At 20k I would check valves, fork seals, oil change, check the air filter-clean it if nessary(sp?) Check for any grease point's and grease em--probably one on the swingarm, a new set of plugs would be good but not nessary, check the wear on the sprockets-(might be worn), go over all the bolts and make sure there tight and torqued, fuel filter, clean and regrease your axles and wheel bearings, check brake pads, not mu h I can think of.
As far as aftermarket sprockets going down one In The countershaft sprocket would be might first move, and cheapest. Then anywhere from +1 to +5 in the rear