Hey Aimless, i havn't done this yet, but there are all sorts of sizes of mikuni carbs that look like they have the same kind of mounting bracket as the stock mikuni. i'm eventually going to get one of those and match the intake to the carb.
i think some float bowls on these other carburetors look a little bit bigger; but maybe you could take care of that with a manifold spacer?
i'm about to get my head milled by a machine shop for about 30 bucks. we'll see how that turns out.
i dont have the simoni pipe though, i have a technigas from a derbi predator scooter. i guess that would make for some differences in tuning.