I would offer this to go 80 mph with $100 to work with...
Buy a $100 helmet, take your bike to the steepest, highest mountain top, and with the helmet strapped on tight...shove off downhill.
I'm sure you're not serious about this...neither is my advice.
If you really want to 80 mph on 2 wheels do what I did... buy a 700cc motorcycle. Going that fast you need a little weight under you and decent suspension at a minimum.
OR.. go see Travis Pastrana
He’ll figure out how to do it.