>Engine-wise, the bus hasn't been used in over a year, so there are
>some issues. It actually fired right up and idled reasonably well
>(it's in need of a new muffler), but it immediately began to cut out
>and lose power under load. Even at part throttle, the motor flat
>spots and runs erratically.

I had similar problems on an Isuzu that sat for several years.  I drained the old gas and filled with new, changed the fuel filter, changed the spark plugs, put some injector cleaner in the tank and let it idle till it warmed up.  By then it was running well enough to risk driving it, so it gave it the old "Italian tune up" (run it at redline in second gear for awhile).  After that it ran well and passed emissions inspection.

If this fails to fix it, then its time to check compression, injector function, fuel pressure, etc, etc....

Bill