You could replace the rear sprocket with a smaller one, or replace the front sprocket with a larger one; either would give you more top end, but have a negative effect on take off, hill climbing ability, etc. I doubt you could get 15-20 mph from just a gearing change, though, without needing to Flintstone it to get moving.
You could build the engine to handle higher rpm and increase power; governor removal, billet flywheel, billet rod, cam, valve springs, intake, exhaust... (assuming typical aftermarket clone parts fit the Lifan 212, which they may or may not; contact NR Racing or another company that specializes in small engine performance)
You could replace the engine with a motorcycle engine; a 200cc Lifan will give you 16 hp @ 8,500 rpm & 11 lb-ft @ 7,500 rpm, a multi-plate wet clutch, 5-speed manual transmission and electric start, all for around $500; a bargain, IMO.
https://www.ebay.com/itm/NEW-LIFAN-...e=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649
You could appreciate the MM for what it is and buy something faster for those times you feel the need for speed.