BlackndWhite, I'm looking at doing the same thing. The only mods to the frame are just as Iceracer said cutting out the down pipe and welding it into a upside down Y then making motor mounts.

You would also need an 87+ carb and CDI box. Best way I've thought of doingit is to cut the frame piece out so you can set the motor in the frame then place the parts to be welded on then remove motor and weld. Its actually a failry simple task IMHO.