Most split screen buses suffer from the same problem when it comes to the heating system: totally rusted away. Being underneath the bus, constantly exposed to water, snow (and not to mention salt!) seems to create the perfect circumstances for your heater pipes to disappear.
You will have to use a grinder to remove the heater pipes from the chassis, since they are welded into place. However, depending on your perception of being lucky, you might be able to just pull them from underneath your van with your bear hands…
The straight heating pipe that runs from front to back is nothing special; you should be able to find a similar diameter at your local hardware store. Cut to the required length, prime the inside and weld into place. Note that I chose to use a pipe with a slightly thicker wall thickness, so it would last a little longer. I guess PVC could do as well if you are not too concerned with keeping it original.
The replacement heater pipes for the front of your bus are difficult (if not impossible) to find and are probably easier to patch up. Use thin cardboard (cereal boxes are great!) to make models of the pieces you will be replacing; copy onto sheet metal and weld into place. Hammer the model into the new sheet metal while welding, as the metal will be much easier to model when it’s hot.
Sandblast, prime and paint the heater pipes after the welding is done. Install the pipes back into their original position underneath the bus from front to back.