SAY YOU BUILD A REALLY BAD A** ENGINE. WILL THE TRANSMISSION HOLD UP? WHAT WOULD YOU HAVE TO DO TO MAKE IT LAST? I HAVE A STOCK ENGINE THAT ATE UP FIRST GEAR

 

TC Racing was the only company I recall who offered beefed up transmission kits for those engines. They were just stock componets that were chemically treated to prolong wear.
Curtis Sparks also offered a kit that offered 6 speeds and deleted the reverse gears but was nothing more than a 350X transmission converted to work in the 250X engine. I dont know of anyone who actually used one however being as how they cost more than a complete 350X engine would have cost back then.

When I was racing 4 strokes in the early to mid 90s, I dont know of one single racer other than Tim Farr who had a beefed up 250X transmission.
And most of the engines in the Pro class were modified as far as from stock as possible....many making the same horsepower levels as a stock 450 today.

Age and abuse is likely what got your first gear. Happens to the best machines out there.