Looks like a gear reduction unit. Though personally I'd take the belt off and replace it with a chain. Belts are know to slip, and if it breaks when you're riding it you might lose and ear... or part of your arm. They usually use these for small kids for safety reasons.
---------- Post added at 07:07 PM ---------- Previous post was at 07:06 PM ----------
Though if you take out that tank bracket, you might be able to put on a TC. Also that GRU.