iso replacement engine for 81 300D

I have an 81 300D (non-turbo) with a blown engine. Research online has shown me that this is a '617.912' engine, common to the following models:

some questions:

1. Do I have any other options other than the 617.912 engine? there are other 617 series out there, would these fit with minor mods?
2. Can anyone recommend a good source for used engines? I remember last year finding an online classified/board of some sort that listed engines by mileage, all were good used compression tested engines. cant find it for the life of me now.

please reply on-list, the email addy I provided is a dupe (I'm spam-phobic ;))

thanks, jamie