I would avoid the type-4 engine like the plague. It is an insanely expensive engine to build, almost as expensive as complete air-cooled flat-6. To get the type-4 reliably even up to 150 hp, you're going to spend about $15,000 in parts alone. Then add machine work, cost of adaptor plate, ...
You can build and M96/M97 much cheaper than a type-4, and the M96 is a great engine. If the M96 cost is still too much, you can swap in a Subaru turbo, or even look into something like a VW R32 engine. The nice thing with the R32 is it's the same bell housing bolt pattern, so should bolt up.
I've got a 76 912, that after some serious research into all the engineering options, I've decided on a 'stealth' Subaru flat-4 swap, with hidden radiators. I'm going with a built N/W subaru in mine for a variety of reasons, but mostly that it's light, simple, lower cooling requirements than a turbo, and I think just looks good.
Here's the look I'm going for.
