Depends on the Porsche Circles. Some places are more accepting than others. Heck, I don't think anyone would call it NARP if you managed to wedge a 928 engine in it. (It's been done, but required major structural mods.)
In local (PCA Zone 7) events, you'd be running autocrosses for NO points; your results would not officially count. ("Must have Porsche engine and body.")
However, check the photos and results from the early Renegade Festivals on the (no-longer-Renegade) 914 Club's homepage,
www.dgi.net/914 . A couple of 914/V8s showed and seem to have been appreciated.
I personally don't care for V8s. I feel that is a drastic change to the character of the car, and not necessarily a good one either. But some folks just don't like a car unless it has gobs of torque.
My own choice would be a 911-based Six, or maybe a hi-po Big Four. The Four won't get you the 275+ ft-lbs of torque of the V8, but 160 honest HP will certainly launch the car adequately.
Final word: Straight-line acceleration is not something the 914 was designed for. You can get it to do that, but it was really made for the corners. If you're disappointed in how the car corners, something is probably wrong with the suspension and should be fixed.
Your car, your call.
--DD