Once you get a rev count, 50-70 watts shouldn't slow it down much,
To crudely find out haw much torque you have, find a solid tyepiont close to the shaft, with the shaft stopped, repete with the shaft stopped, ie in gear, rap a rope around it, about 3 times, keep tension on the other end, disengage gear, slowly lessen the tension until the shaft spins, chech tension in the fixed side, if you have avalible a spring scale even better,
Warning don't do this until you understand it!!!!!!
As for pulley size, what size is the pulley on the alternator, I can throw numbers but without your numbers they won't work. I would go with the largest pulley I could get/fit/afford, then change the one on the alternator as it is likely the easiest,cheapest to change