Grand Marquis GT

Hello, I did a search, but didn't find much. I am usually a chopper fan (yamaha engines also make for great old school choppers!) but I just picked up a 1991 YZ125. Got if for free.99 (Great price, yeah!) but the top end of the engine is apart. I've read that the 1991 engine isn't so great, so doing a stock rebuild may be a last resort. How easy would it be to put a later model engine in there, possibly even a YZ250 engine? Thanks in advance