1project, that is the heater out of my tournament ski boat. I couldn't find a replacement core or a heater that was close enough to the same, so I used a core from a vehicle that is common and added onto the heater box so it fit inside. it would be easy to fab a new box to fit around a common core size and install a motor/blower fan, then cut out some outlets that fit the vent tube diameter you need and install some fabbed up vent tube adapters into the holes.
https://www.summitracing.com/search/...ing-components
there are lots of small heaters with pretty good output for defrosting and/or heating the cab. depends on what you want. if it's for a full on winter day, maybe something with more BTU's and outlet controls is needed. if just to clear the windshield then a smaller heater may be fine.
google search for cab heaters should bring up a bunch. tractor heater, bus heater, UTV heater, etc.try the google images once you get to the page.