The hard part is in varing the speed.  You can try something like a
variable pitch sheave.  It is a pully that opens and closes to vary the
speed by about 30%.  Most of the ones I see in catalogs use a set screw or
flange to set the speed one time.  You need to find one that can be
adjusted while on. Good luck.
 
Lovejoy 3410 Spring loaded pulley - 3/4" Bore Ratio is 2.1 -3.8 with and A
belt or 2.2-3.7 with B belt.  Cost  $108.57 Canadian (~$80 US)
 
I built 48 frame radials for use and sale for a few years using them as a
transmission.  Worked beautifully.
 
Allen