If it's the 12/12 or 12/10 carb it's a 49.
A clogged air filter would make you run rich. Four stroking sound like a slightly anemic growl, where your 2-stroke should sound like ziiiiiiiiiiiiiiiiiiiiiiiiing, really high-pitched and all.
Keep the multivar, it rocks and it's rockin' will become much more apparent when your engine is making a bit more power.
I have the 10mm DMP crank from Zippy, and so far I'm quite satisfied with it. I sheared a TOP crank from treats, but I think that had more to do with a fractured flywheel than a bad crank.
You can probly get the local welding/machine shop to brace your swingarm for you. Youll want to do it if you start hopping up the engine. It's not really the bumps that will crack it so much as engine torque, and that light gauge steel is just not up to a moderately powerful modded engine. Not like you have to do it immediately; just keep checking your RH subframe from time to time...you'll know when its time :)