How difficult is it to change the belt on the motor to countershaft pulley drive? If things are reasonably accessible a quick and dirty expedient is to mount two sizes of pulley on both motor and countershaft chosen so the pairs give you appropriate high and low speed ranges. Get a two belts, one for each pulley pair, of lengths calculated to give very close centre distances. Generally a variation of less than 1/4" or so can be arranged. A reasonably hefty spring in the belt tensioning arrangement will accommodate that or you could go all posh with a spring loaded jockey pulley. If you decide to go for a jockey pulley it would be worth investigating whether light modification of a car fan or camshaft drive belt tensioner assembly could suffice saving on building work.

Although it sounds bodgey the two belt system worked well enough for me in one application for several people to copy it. Second time round I did it properly and worked out a set of pulleys to do the job with a single size of belt. Even with computer assistance the gain was not, objectively, worth the headache.

Clive