Not sure exactly what engine you are working with, but 2100 red line is the norm for most DD 2 strokes found in late model buses.
Most -71s and -92s are set at 2100. Some transits are set at 1800 for fuel conservation.
Unless you are running a -53, or some sort of vintage DD, 2100 is a reasonable red-line.
They can be turned up to 2300, but that's really wringing the engine. Could be rod throwing territory.
An 8V71NA, or any other non-turbo'ed DD isn't going to be fast at all.
Transit engines, even with turbos, are slow too...because they are turned down.
The trick is to downshift when the top speed of the next lowest gear is reached...
Figure out what red-line is in each gear. Then, when driving the bus, if you see a hill coming up, get up a good head of steam before you reach the hill, then once the speed starts to drop off, downshift as soon as the next lower gear max speed is reached. Don't dink around...gitrdone quickly. Drive it like you stole it!
A turbo engine really benefits from "spooling up" before you hit the hill. Once your speed falls off, it's hard to get the speed back up with a 2 stroke.
No matter what you've heard about DD 2 cycles, 8V92TAs are fast....all other DD 2 strokes are not so fast.
What engine do you have? Post some pix of your coach.
What burned on it?
The fuel return lines allow the large volume of fuel flow to cool the injectors. They'll run without the return line as long as it's blocked off. Probably don't do the engine any favours. A leaking injector would likely be quite obvious if the return line is blocked off.