The title pretty much sums it up. They show different part numbers but I am not sure if because of the elctric start and the battery on the 300EX that they will interchange. Any help would be appreciated. I have a 1987 250X

 

I would say on a stock 250x, no.
The 250x system is a/c and not rectified to dc, like the 300ex is. If you could swap out the wiring harness, and add the rectifier, then it would probably work.

 

My 250x has a regulator/rectifier on it

 

If its a stock 250x, then its only a regulator. If It only has one plug on it, and it has a white/yellow and a green wire, then its a/c. If it has 5 wires going to it, like the 300ex, then its d/c.

Right now, there are a few 250x and 300ex regulators on ebay, see which one you have, by looking at the pictures. Then look at the cdis for sale there too,

If you try putting the 300ex cdi on the 250x, let me know how it goes.

250x volt reg
300ex volt reg/rect