[rotary911porsche]

Why put a piston engine in? It's your car. Do whatever you want.
The V8 does change the weight balance a little but it is very easy to get back to 50/50. The 20B would probably throw the weight off just as much. Have you priced a 20B? I did for my conversion and the total with wiring and engine management etc. was going to be over $8,000. A strong V8 is much cheaper.
There are several places to purchase a kit to adapt a Chevy or Ford small block. It really is a bolt in installation. If you choose to retain the fuel injection of the V8, the wiring will be the hardest part. But there are companies (Painless for one) that will mod your harness and label everything. It's not cheap ($300 to $400) but it is easy.
Changing to an automatic tranny makes the job even easier. Adapting the Mazda clutch master cylinder to work requires some creativity. But many conversions are stick.
Again, it's your car. You have to ask yourself; "Do I really care if someone else thinks I violated the soul of MY car?" If you do care, keep it rotary. If you don't than make the swap.
One note of caution. If you plan on racing SCCA an engine swap will change your category. It is a major mod and you will be racing against serious track only cars. But if the car is for fun it is not a problem.