New to the site, thought I could pick some experienced brains here a bit... I would appreciate any help.
I picked up a bike from my mother's estate, it had been outside for 20+ years. Seems the rubbish that collapsed around it saved a lot of it, I currently have it running with a new IT400 carburetor. 
My problem at the moment is that I had popped it into first gear when I was picking it up, and now it's stuck.
I did a case fluid change (synth 10-30 with Marvel MO) when I got it back to the house, and let it set while I got the engine running. Ran the engine about 15 minutes then opened up the clutch cover to unstick the clutch plates. I saw no obvious issues on the shift lever on that side. Sealed it up and filled with castrol dino oil 10-40. 
When I try to shift into neutral, the shifter seems to go the entire range but doesn't catch. I'm thinking that the shifter pawl got stuck and is not returning to catch the drum, if I'm understanding this transmission at all. 
Question is, will the pawl loosen up with fresh oil eventually, or will I have to split the case?  Or is it a worse problem entirely?
 

Gonna have to go back in the clutch side i think. Could it have popped the circlip on the left then shft moved over & ut of mesh with the thingy it runs with. Not sure you'd get the case back on like that, but maybe there's plenty of clearance as its all hald left by the circlip. I guess gear lever & black spacer would need to be off the left side for that to happen. But while back in the clutch side revolve the drum to be sure its not rusted up if its had water in the trans. Take the plunger & spring out from the left underneath to make the drum easy to turn by screwdriver but you'll need to either manipulate the mainshft or sprocket shaft to go though 5 gears although it won't stop in gear exactly without the plunger n spring. So, number 9 in the link must mesh equaly on the teeth of its mate. & be sure the lever return spring is ok, because if its been off its peg then the legs get crossed rather than parallel over the peg. Pic coming.

 

I see a circlip both ends but was reffering to the gear lever end. But then again it depends what you've been up to there as either circlip would give the same result if removed & things have got outa mesh. Ok is an MX400 pic but should be same horse. Just wasn't a DT on Partzilla's "75 page.

First thing to check is the sector gear timing/mesh.  May have jumped a single tooth from a hit on the shifter or something.Alignment marks highlighted in red.

 

I will check those that out, thanks for the diagrams and pics all.  I have the manual in digital format, but its hard to follow without a starting knowledge of the bike.  I appreciate all the help, hopefully it's that simple.

So it seems the culprit is the pawl return spring, if that is what it's called. Broke at the pawl. 

Ah, ok. For sure that'd cause it to go through the full movement without pulling on the pins as you said [& i didn't think of that as never had a spring break]. There's always something new to learn eh!!.

Did the spring break because the selector drum is seized in place ?

I didn't get a chance to check it out completely, but it looks like the spring corroded right at the bend around the pawl. The drum slipped into neutral easily after I removed the part, so I'm guessing it is probably ok. I'll be able to get a better idea this afternoon. ​​​​​