RE: Kyosho TUrbo Optima Mid SE Questions.
Cool! I used to have one of these, but sold it years ago. I'm sorry, but I can't help you much on your questions. I really do think ebay might be your only decent parts option, unfortunately. Either that, or find a Kyosho forum with vintgae guys buying/selling vintage kyosho stuff.
I never ran foams in mine (they didn't have 'em back then!), so I wouldn't sweat it. Either that, or buy some 2.2 foams and trim them down.
I do think that a MM 7700 is way too hot (meaning, too fast) for any 4WD offroader, including this one. A more suitable choice is normally a 5700 tops for offroad 4WD. So I would not get anything faster. In fact, I'd be worried if the belts could take either setup (5700 or 7700).
Either one will yield insane speeds and power.