Ive had the worst luck with the 350X I have 3 complete engines that need top ends
If your lucky it will just be a valve adj.and worse case scenario would be what the goat said unfortunately thats my problems the people that had all 3 of my big Xs
either put the oil filter in backwards and starved the top end of oil ,or just plain run it without oil, or oil pump failure I think its doubtful for oil pump failure .
83 YTM 200,84 200X,,85 350X basket case,86 350X parts trike,86 350Xroller needs top end, 86 250SX,