I have a 1981 IT125 that is like new, except age has caused the gas tank seam connecting the top to the bottom to split.  And I can't find a replacement tank specifically for an IT125.  I have found tanks for IT175s.  Are they interchangeable?  Does anyone know of an aftermarket tank that will work?  Or is there a "fix" for the split seam?

Have you checked Clarke Manufacturing? I know they make a tank for other models in the IT line.....

They have a tank for a 175, but don't show anything for a 125.  That's why I was asking if a 175 tank will go on a 125.  I would think they should be close, and in looking at parts diagrams they look very similar as far as mounting brackets etc.  I just don't want to spend bucks without knowing it's interchangeable.

Doesn't look like it will.
The IT 125 part number is : 3R9-24110-10-00
And the IT 175 part number is : 3R6-24110-10-00
Close tho....
 

the 125 had a 8.5 liter tank
The 175 had an 11 liter tank.

The early It 175 tanks fit but look odd and i had to adapt the tank base fixings to get it to work... until i was fortunate enough to acquire a alloy IT125 tank that came up out of the blue on ebay. i do see the odd 125 tank in the US on ebay but they are usually in bad shape.
my original went in exactly the same place and i had it plastic welded by a guy and it did hold fuel . i later sold it to a guy in the US and ebay destroyed it as it breached their postal regulations!!!!!
frrom memory i think it was a 78 IT175 tank i used
it maybe possible a 1980 YZ100 tank will fit but they are pretty rare too....and white but you could wrap it!
    

I have found someone who does plastic welding.  He is looking into the type of plastic used in these gas tanks.  It sounds like you had good luck with plastic welding?  There may be hope yet.

I did find out the reason it split was because the breather was not venting on the lid as it was clogged, thus the tank contracted in the heat and then when it expanded it just split the tank so always worth making sure the lid is venting or just unscrew it and drain fuel if your parking it up for the foreseeable future

In this case the vent is clear and the tank sat empty for 30 years, so I'm more inclined to think it's just age.  But what you say makes sense.  Age, plus extreme temps could still cause a problem.  We had 30 degrees below zero(F) here in Nebraska last winter.  That's cold enough to cause all kinds of problems with lots of stuff!

After some research, it appears the 78-81 YZ100s used the same frame as the IT125, so a YZ100 tank from those years should work.  It will just be yellow instead of white.  There is a decent looking one on EBay.  I'm hoping to hear from my plastic welder soon about whether the IT tank can be salvaged.

Thanks for taking the time to update the post!

Well I got the tank back from the plastic welder, and it is holding gas and I can ride the bike.  It's not the prettiest, but at least I can say the bike is still all original!  I do have a concern about more of the original seam failing, but will jump off that bridge when I come to it.