Here's my 300x.
Years ago I was told yeah 250x 300ex motors swap right in. Never saw one in real life or on a forum however. Since I hate job I needed to get my mind off things.
300x project started
YES a trx250x / 300ex motor a FITS in.
NO a trx250x / 300ex motor does NOT bolt in.
The front lower 350x motor mounts and rear lower mounts line up and bolt in 100%
The head stay mounts are not used.
The middle lower bolt between frame rails lines up and bolts in however different width spacers are needed.
The rear upper motor mount lines up but does not bolt in. You need an extra 20mm on that bolt. 350x is 140mm so you need 160mm long bolt. Belmetric or McMaster-Carr have longer bolts.
The rear motor mount that bolts onto the frame needs to be formed opposite (mirrored) of the 350x oem mount. The oem length spacer needs to be made longer.
You could mill half the 300ex rear mount off on opposite side of motor just as guys did on 350xs. But that is not the correct way of doing it. Just make a new mount or you will crack cases. Any metal forming / laser cutting place can do this for you.
I have the specs to produce the correct spacers for the swap.
Lastly here is the start of my seat latch base. The sheet metal was junk. Most of them will be by now. I had laser cut this piece to oem specs. However between 3 frames not one had the same dimensions. The old frames were just build +/- 1/16" at least it seems.
If doing this swap please fully gusset your frames, if you think nearly 40 year old paper think frames are strong I'd suggest not attempting this type of build.
Stay tuned. This should help people put.