Here's an example: a snarling 2-liter Ferrari V12 from the 1952 166 coupe. There were a lot of hot little Italian engines from 500cc to 2000cc in Italian cars from the '40s through the '80s. Probably some sitting around somewhere.
Subaru engines kick ass. A popular swap these days is a Subaru flat four into a an old Porsche 914 or 911. A WRX-STI motor can be tweaked with a chip to 450 hp. There are a lot of VW Vanagons running around with Subaru flat fours and flat sixes.
A sport touring rig w/ a longitudinallymounted 4 cyl. Specifically, the 2.3L Ford that was in the SVO Mustang and the T-Bird Turbo Coupe, with a tranny from a MotoGuzzi. Kinda like a Griso, but w the 250-300 HP of which that motor is capable. Even a mild tune of 200 would be kinda cool. Big rear, long wheelbase, comfortable seating position, and the ability to bust the back tire loose at will. Also, no muffler other than the turbo, with a little downpipe.
This may not be the most imaginative response, but I think that the engine in the 2007 Honda Civic might work nicely. I makes good horsepower and torque at relatively low rpms compared to current motorcycles and I don't think it would be too large to be practicial. Here are the specifications I pulled off of the Honda web site.
I vote for a Mazda Rotary, like the current gen from the RX-8. Even if it's scaled down, a compact, low torque (for a car), high revving, high power-to-weight engine seems like a no brainer for motorcycle applications. In fact, if you optimized one for a bike it would probably rev even higher than it already does.
Piston engines in bikes are more oversquare than in a car, sacrificing torque for higher revs and more horsepower. I wonder what the rotary would look like if it were built from the ground up for a sportbike. Too bad my engineering background is in aircraft, not engines, though I still think it could make an interesting project/hobby.
The rotary engine has come a long way since the last rotary bikes died; it might actually have merit again.
I was being silly at first, but how about just one cylinder from a turbo cummins diesel? 1000 cc, 100 ft pounds torque, 50 horsepower, 4" exhaust pipe, and a jake brake, 350,000 miles, give or take, between rebuilds. I could live with that. Maybe we could get Kenworth to build it. Been on the interstates lately? If ya can't beat em, join em. If we was willing to sacrifice a bit of engine longevity to get more horsies, maybe a one lung cummins could put out 100 ponies in exchange for say, 175,000 miles between rebuilds., I could live with that too.