You haven't given us much information on the parts you're using.

If your new piston has the clip grooves machined closer together to fit the new pin, that would be fine... this might be likely if you found a "piston kit" from a manufacturer that came with piston, pin, rings, and clips.

If the new piston has the pin clip grooves machined the same distance apart as the original piston, I would get the correct pin.

I couldn't find a kit that provided the piston, clips, bearings and pin altogether so I bought everything separately. The piston I purchased from a seller on ebay mentioned its a genuine Yamaha piston. (picture attached)

 

Im using the original piston pin clips, which seem to fit fine within the new 1st oversize piston.

The piston pin I purchased from a separate seller listed it as the same part number (214-11633-00-00) I pulled from the genuine parts catalog.
Here is the link to the piston pin I purchased .

I reached out to him regarding the 1/8" size difference between the one he provided and the original. He mentioned a size difference like that would be inconsequential. Rather than take his word for it, I figured I'd run it by the pros here 

Overall, the fit of the new pin seems right, except for the 1/8" difference in length, which I wasn't sure would eventually cause uneven wear or eventually cause a failure.
 

Just curious if piston and pin came in Yamaha packaging? I've found many sellers show a picture of Yamaha packaging but ship something else. I once got a low quality Chinese piston that seller advertised with a picture of a genuine Yamaha box.

Here's a picture of the 214 pin with dimensions from Partzilla, maybe that will help determine if you got something "close" that a seller is trying to pass off as an original Yamaha part? www.partzilla.com/product/yamaha/214-11633-00-00

I wouldn't use the shorter pin but I can't tell you for certain that it would create a huge problem or anything.

I was taught to never reuse piston pin clips. I never do. It's not worth the risk. If you've ever seen the damage caused by a clip that came loose, you'd know why. Also, gap in clip should always face up or down, never to the side.

Thank you @MarkT!

The pin came in a plastic bag and the piston came in what appeared to be original packaging. By like you mentioned, you never know if its cheap knockoff. The seller did designate "no returns" for this listing... Out of curiosity, how were you able to tell it was a fake piston that was sold to you? I compared this oversize one to the original and it seems to be the genuine. Just the internal finish seemed a little more rough than the original and it isnt stamped "Izumi" on the inside like the original. (pictures below)

 

I initially did reference the Partzilla site just to double-check that the part number was in fact for a DT2 250, before ordering. 
From the picture they provide you can eyeball that the pin is 2 5/16" - wish they wrote the exact measurements somewhere in the listing.

I tried calling and their support system is going through an update 

Ill take your advice and find some new pin clips and a pin that matches the original specs. Thanks again!

Piston looks fine. IZUMI was an OEM piston maker.

Partzilla sells the pin for about the same price as the Buzzard pin. Bike Buzzard alternative stuff is often "close" but at least he does clearly say when it's not OEM. He sells crank seals that are the right size, but not the correct dual-lip design... the seals sometimes work but aren't as good as OEM Yamaha seals (which can be hard to find... but Enduronut or DEET might have them... next piston you need, check with Enduronut, probably cheaper than ebay and he's an expert on Yamaha parts)

You might also try RMATV-MC. They have the OEM Yamaha pin and clips in stock. If you spend $75, shipping is free. RMATV service has always been excellent for me... I usually get the order next day to Southern Nevada.

Partzilla is hit and miss... and they shorted me parts the last time I ordered a few years ago... their packing list noted the shortages... but they wouldn't refund or ship the missing parts so that was the last time I ordered there.

Link to RMATV www.rockymountainatvmc.com/oem-parts/Yamaha

.
Eyes sometimes deceive us . . . The piston IS genuine Yamaha, but made by ART
What appears to be the word IZUMI, is actually a model number . . . 1M1-01

So, what we have here is a later model piston that has had the top ring land machined to accept a Dykes style ring
( The 1M1 piston did not have a dykes top ring)

I hope it was machined to match the 438 piston specs,, and that the stock 438 ring fit it correctly.

 

Thanks for sharing your expertise Dave! You're far more more an expert on these pistons than probably anyone on earth.

I could swear I've seen original Yamaha pistons with an IZUMI casting mark but I certainly could be mistaken and it was a number. It most likely would have been a 175 piston as that's where most of my experience lies. Sometimes my memory plays tricks on me too.

I have the same bike and mine sounds almost identical to yours. You can listen to mine and see what you think. If it's not the same yours might be a little bit better but it's darn close. I have no plans to replace the piston or enlarge the bore. The rattle adds depth of character. But to put my opinion in some context, I still have the original tires on and while I do plan to replace those I don't let 50 year old rubbers stop me from enjoying the bike in the meantime. Tire options are very limited and mine are still holding air so it isn't a budget priority. 

Nevermind. I zoomed in on photo and definitely see the "Art" brand casting in this piston.

If it's an ART, it can't be an IZUMI which I'm still pretty sure is another good manufacturer.

All makes sense now.