Stock sprocket combos, of 3-speeds: K0 -15/38, K1-later 15/35. Both work out to 4.77mph/1000rpm in top (3rd) gear. Don't worry too much about the hardcore tech side of this. I'm trying to explain just how precise gearing has to be with these little engines. At most, you can go one tooth smaller at the wheel. That would give you 3% taller gearing. Any taller and the engine won't have enough torque to pull normally in 3rd gear. 37 & 34t wheel sprockets don't exactly grow on trees...sourcing either would be a bit of a hassle. And you'd only gain 1mph for your trouble...maybe.
Seems far more likely that the motor is down on power...waay down. A healthy stocker should top out in the low 40s, over flat terrain, given a long enough runway. Yours is coming up ~30% short which, by the numbers, indicates an engine that is delivering roughly half its stock horsepower. You're right, 2000 miles isn't much; these engines should deliver several times that. But, lack of maintenance and decades of improper storage can take a heavy toll.
A clogged air filter, dirty carburetor, improperly gapped points can choke-off an otherwise sound engine. Worn sprockets, a rusted/arthritic chain and even sticking brakes can all scrub-off mph, as well.
You probably have some combination of the above issues. Despite coming across as a large-ish grocery list, it's really not that tough to work your way through. I'd start with the point gap & air filter. Resetting points is free. Air filter elements are cheap. Next up, inspect the chain & sprockets; if they're worn, replace them...you'll have to do that anyway. Next up would be a carburetor rebuild. If it's still not running up-to-speed (literally) time for a compression check; less than 130-140psi, indicates that it's time for a valve job at least.
