They got bought up, hopefully production will start soon. Worst case the Chinese will gladly make copies, pretty hard to mess up making a TAV if you are set up to stamp sheet metal.
You CAN run a 50 amp welder on a 30 amp plug, just not at full capacity. Worst case you will pop a breaker, in which case you need to run downstairs and reset it so the cooling fan keeps running.
Drum brakes are found in the back of cars because they are cheap and get the job done. Look up brake fade, you will understand why disc brakes left front ends by the early 70s.
Don't replace the belt with chain, just get a smaller pulley for the trans and/or a bigger one on the engine. I am in the middle of building pretty much the same thing, but in trike form.
You might want to tear down that diff and if it is in less than perfect shape, weld the spider gears to the pin as a crude "lincoln locker". They tend to come apart in normal mower use, let alone abusive kart applications.