The only real reason for upgrading your rear axle is because of shifting gears. Normally people will have 1st gear at a frame twisting axle bending 30:1+ to make take off real easy and help with super crazy hills. If you run a single speed with normal gear reduction around 16-20:1, you won't have that big of a torque multiplier to the rear.
well my motor definitly isnt stock. it has the cnc machined high compression head, port and polish and the higher power lightning spark system. so im guessing itll be putting out more power then normal. one other thing im wondering about. the fellow i purchased the bike from installed front and rear disc brakes onto it. the front disc is mounted on the front wheel hub and the rear disc is mounted to a cnc hub adaptor along with an original 46 tooth sproket. the calipers however are attached with some sort of clamp that wraps around front fork on the front and around one of the rear wheel arms if i remember correctly. now i realize this probably isnt an optimal setup. but would it be good enough brake system to use for this application? right now i see it as my only option since i cant use a coaster brake with the jack shaft and the bike had no front brake originally
Those brakes should be just fine, those are adapters which technically speaking aren't as "solid" as calipers mounted to a frame designed for them but it's more than acceptable for your uses. Personally I'm running 30-35mph cruising speeds with only front rim brakes. You'll be fine with dual disks.