the ecm part #'s help...there's about 80 results for the 4770 ecm on ebay now. even one "new" unit in the standard box for $47.50 OBO.
search results for 1224770 ecm =>
1224770 ecm for sale | eBay
and there's about 50 of the 4810 ecms on ebay link =>
1224810 ecm for sale | eBay
i did not start to sort thru the listings looking for prom codes. perhaps its a start.
also thought the way to get the prom you want would be to find someone who has one and duplicate it. many decades ago I used a Needham Electronics SA20 (E)eprom burner. there are sa20's on ebay, but that's a path maybe for later.