You will not likely go wrong with that engine and that transmission.
Somebody else will tell you what a "Crown/Spartan" is -- I have no idea.
But mechanical parts on buses are pretty much standardized throughout the truck-and-bus industry, so when you need brakes or a radiator or whatever, you can easily get all that. Let's say you need a windshield wiper motor. You would not need to buy a Crown/Spartan windshield wiper motor from a Crown/Spartan dealer. You would buy a Leece-Neville, or Peterson, or Whatever windshield wiper motor to match your old one, from any truck dealer or auto electric shop. No big deal. Engine parts -- Cummins dealer or any truck parts shop. Transmission parts -- Allison or....