Like a lot of the odd and awesome stuff one encounters on the internet, there's tanitalizingly little information available on the Polimotor. From what I can gather, there were at least three different engines developed using the technology. I'll start with the two that appear to be best-known.
"Designed and developed by Polimotor founder and president Matty Holzberg, the engine, based on a Cosworth BDA, tipped the scales at 168 lbs - half the weight of its metal counterpart. Plastic parts included the engine block, cam cover, air intake trumpets, intake valve stems, piston skirts and wrist pins, connecting rods, oil scraper piston rings, tappets, valve spring retainers and timing gears."
Illustrated below, and powering the Lola T616 pictured beneath, this iteration developed 320 HP at 9,500 RPM and ran reliably for two seasons of IMSA Camel GT Championship in 1984 and 1985. Here's a link to the quoted source.
Here's a Ford 2.3 version - it made 318 HP @ 11,000 RPM and redlined at 14,000. Popular Science further details it here. There seems to be some confusion as to whether these two are the same motor, as the articles linked above, in addition to this Wikipedia entry, contain contradictory information.
The following is quoted from an old Car Lounge thread, itself quoting a now-dead link:
"The engine used metal cylinder sleeves, metal combustion chamber tops, metal piston crowns, bearings, valves & seats, and a stock 2.3L Pinto crankshaft. Darn near everything else in the engine, including the block, conrods, piston skirts, etc. was fiber-reinforced plastic. The exact type of plastic escapes me at the moment. Very little metal was used outside of the crankshaft; just small/thin metal parts to shield against direct contact with combustion, and on mechanical wear surfaces.
It was also noted that the crankshaft could have been made of fiber-reinforced polymer with thin metal journal inserts, which would have upped rpms/hp even more and saved a bunch more weight, but they stuck with the stock metal crank because of budget & time limitations. They were really pushing the technological envelope as it was."
The third, and perhaps most interesting of the Polimotors, is the turbo V6. It seems to have been developed for Indy racing, but I don't believe it ever ran in competition - I'm sure there'd be more details available if it had.
I'm utterly intrigued by all this, and frustratingly left with more questions than answers - though in some perverse way, the mystery makes it that much more satisfying. I hope your mind is blown even half as much as mine was.