I'm not sure whether the diesel heater poses a problem with passing safety inspection. I do know it probably isn't the best idea to have such an item unsecured in the cabin.
I honestly think you would be better off putting it back like it was. Hopefully you kept the parts involved. I would replace the heater core itself since you already have it apart.
If you have tossed the original parts, you might check with a salvage yard to see if they have one with the same setup. Sometimes bus bodies are utilized for storage for engines and transmissions that have been removed from vehicles, and chances are good they do little other than pull the body off the frame and remove the engine and running gear for resale.
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)