Youre gonna need a welder,,scrap steel and some skills but it can be pulled off...The other alternative is downsizing to 185/190 I hate to say....The older mini-quads used to be able to handle riders up to 265/270 easily.....
I had to mod my own Jetpro exhaust I bought for my piston ported cag to fit my reeded engine cag....Theres only a few bikes they just install into with minimal mods to mounting area.......I'm currently doing a Zstar build, and have the same flimzy looking axle... You can have one machined from a solid rod of carbon steel, have it polished and treated, at a machine shop with reverse engineering equipment. I did it myself and basicly payed only for the steel, but it would probably run you up 300-400 dollars.Also, PBU sells an 8" machined solid steel rear axle for 109$, I'll post the link for it below.
PBU also sells all kinds of performance kits, but they are very expensive, and you can probably match it for half the price with a 3 port billet head kit with 12mm wrist pin, some better reeds, a Full circle crank, a good pipe, and a Del'orto replica pumper carb... plus a few home made mods that will basically maximize the potential of what you have at hand.
I'm barely 275 dollars into my build and have most of whats listed above, aside from the pipe which I just started cutting today, since Jet Pro pipes are not very quad friendly, lol.
anyways, here's the link to that axle:
http://www.pocketbikesunlimited.com/Q4-PERFORMANCE-PARTS.html
I got vids but theyre taken on my crappy phone cam a few years back...Lots of my vids became corrupted beyond repair....;-almost 13 gigs of pics and vids..............Lately i havent been bringing the vid camera out..They get busted real quickly and it takes time away from the fun............Off subject.... CAM, Your RC cars are off the hook!!! Need to see one of those roll! Do you have a video of any of them??