I found a nice selection of Fastar heaters on eBay:
https://www.ebay.com/p/12l-FASTAR-28...ess/1172948539
Photos of your installation would be nice. Mounted inside the box, but isolated and vented so it does not use air from inside the box, correct?
Regarding the Shurflo 4008, does this provide a nice pressure; comparable to mains in a home?