I don't have experience with that type of conversion to a 912, but a few things I'll throw out:
VW Type 1 1600 based engine--If you babied these engines and are diligent about oil changes, valve adjustments, etc. in stock form you "may" get as much as 100,000 miles but not likely. Once you start modifying them the life goes out the window, but a lot of fun for 50,000 miles. Most everything is available with the best components, aluminum case, nikasil cylinders, etc. Nowadays you can build a very strong stroker 2L. (100-150hp) engine for about $5000-6000 (like anything though the sky is the limit). These are the absolute lightest air-cooled 4 cylinder engines though, ~250lb. fully dressed stock--lighter is possible.
Precision Alloy Ltd.
http://vwparts.aircooled.net/Aircooled-Net-Mini-Stroker-VW-Long-Block-p/mini-stroker-long-block.htm
Type 4--Jake Raby, 'nuff said. Expensive with the shroud conversion, manifolds, and motor, but if you watch thesamba.com occasionally deals pop up. The cylinder heads and dropping valve seats are still the main thing to watch for, other than cost here. About 350lbs. and at least 100hp.
Polo Motor--About $20K. Yikes, but they're upwards of 200hp. Everything 911 in a 4 cylinder package.
Polo Motors