Now remember as I'm keeping the car in vintage racing trim I'm not really seriously considering it BUT if I ever did:
Yamaha R1 motors can be had for $1000 US all day long, the older FZR's can be piked up for $500. Both motors will easily put our 175HP, a revised airbox and exhaust makes a huge difference to the engines. My coupe is currently at 1690 lbs with the overbuilt cage. The R1 motor/gearbox would drop 60 lbs off the car, with it no longer needing to meet vintage rules I figures it would be possible to get the car down to 1540 lbs (I've weighed a lot of parts) . If you fab a coupler assembly for the counter shaft sprocket you could use the stock driveshaft. I'd run with out reverse (just by an old honda quad or 3 wheeler for $400 for the paddock)
When we beat this up, doing a bunch of the labor myself (I don't weld or machine parts - well at least not properly) but paying for the important stuff. You're looking at $4-6K in labor and about $1500 in parts, $7500. Now that's still more than the car is worth but it sure would be fun. I've attached or attempted to a picture of our old bike powered sports racer (black car passing the yellow one) The car was completly hand built using lots of production parts that were then custom fabbed. (The picture is in the photo album under track/racing titled bike engine car so if I screw this up somebody please attach it).
http://datsun1200.com/modules/myalbum/photo.php?lid=19366&cid=18)
Tom