For the cost, labor, and frustration involved, you would be much better off buying a bus that already has a diesel engine rathing than trying swap for a diesel.
as long as it's still running, you can always sell your current bus for more than the $450 you paid for it.
I only paid $775 for my bus (1991 Ford B700 72 passenger ward body with a brazilian made 6.6 litre ford diesel automatic allison trans) and i've driven just over 13,000 miles so far. The only big mechanical problems i've had to pay for: Rebuilt starter $200, and a new water pump $200.
Any of the DT series engines (DT360, 366, 466) are supposed to be excellent, and cheap to work on. From what i've read, the DT466 is probably the most coveted skoolie motor that's out there.
Quis custodiet ipsos custodes (who will watch the watchmen?)