Here's what I do. Place a 2x4 across the fenders above the generator. Get the generator about where you want it with friction from the nut holding it in place. Use a light duty chain or rope to loop under the generator, bring it up over the 2x4 and bolt/tie it on top. Then a pry bar on the 2x4 under the chain/rope will gently bring it the rest of the way up to where you want it.

Remember you don't want it tight - keep 1" belt deflection on the longest run of the belt. Too tight and you'll destroy bearings in waterpumps and generator.