Hahaha tell me about it, riding the bmx downhill fast it definitely beats you up! I might put a Marzocchi fork and a disc front wheel I have sitting on it if my wrists get chronic. Rear suspension helps too since I was using rear pegs and my knees were taking all the shocks. Its so easy to keep putting things on a bike though so I'll prolly keep this as my simple BMX.
I wanna make something real mellow with a four stroke, and if I can afford one I think I'm going to use a ghx50 clone. I'm tempted to use the bigger predator engines, but like you said about power to weight ratio, might still use a predator if the frame allows it. There are a lot of neat transmissions that work with the lil four strokes, you really have every choice in drivetrain type, from chainboxes to gearboxes to cvt's to right angle drive.