you'll find 2 of the 3 heaters
in the accessory manual - if it's not one of the 2 then it's the "standard" factory installed recirculating heater which is very easy to figure out [only case, core, fan]
your kinda stuck without the original core tho' cuz you need it to find the size, and the core is actually only the finned central part, seperate from the end tanks, which have the hose fittings mounted in the correct place to go thru the case and firewall [and prolly got turfed w/ the core]

nobody reproduces the actual cores, you hafta take your old one in to get redone
need to find a used original of the correct model to get the pieces yer missing - if you can post a pic of parts you have I can tell you which one you've got - or just start searchin for a complete heater, like in the swapmeet section - possible some of the vendors have complete reconditioned heaters too [like Carter for one]
Bill