I've ridden the 660 before they turbo'd it. At a demo ride. Not a lot of power on it.
As for the mods, again I will say that there were mods available in a limited amount and sources. They are gone. The sleds are dead now. Keeping it in pristine original condition would be your best bet if you want to preserve the history of it. Mods when available only added a limited amount of hp and they required better gas or it detonated badly was reported. These engines were car engines in a third world country basically.
Again, want more power or a better sled ? Buy an 800. That's better. The T660 was a very heavy machine that was outclassed in many other ways.