The heat system in the buses are hydronic with fan forced heat exchangers.
You would need to heat the fluid (usually anti-freeze) with a heater (think water heater.... like an old 6 gallon LP Bowen/Atwood) and put a pump inline that is for circulating hot water/fluids (like an El Sid or similar). You can keep the original heat exchangers and build a duct system to distribute the heated air. Hey, I just described how we will do our heat system.
Substitute (or include) PEX lines under the floor to the system and insulate . Look at outfits like Radiantec
figure out what type of system you will install and download their DIY manuals
A closed system is the easiest to install but will require a dedicated water heater. You can install a valve so that you can use the heating system to preheat your diesel engine or use the engine to heat the fluid while traveling down the road (when you can't use the water heater). You are building your own Aquahot
. Aquahot manuals