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So I wonder, what kind of motor would produce, say, 100 hp, and be small enough so that it would've fit into the original Seven
The same engine that's in my Formula Ford. A Ford "Kent" 1600cc pushrod motor. They came in Pinto and Fiesta back in the 1970's. They are silly small by modern standards, 14.5" bottom of block to top of valve cover with a 6" wide head. On this motor the history of modern engine development was founded. First the Lotus Twin cam head was put on, then the Cosworth Belt Drive head. Upon those was born the Cosworth DFV... All in the space of just a few years in the sixties...
If you have access to these pieces, it makes for a very nice package and the history is incredible...
Some of the original GM V6 pushrod ( 2800? 60 degree banks ) would fit, I think.
I think mmraie and rumbles are correct above. The turbo motors are expensive, heavy and have enough torque to require the same drivelines or heavier than their V8 counterparts. The focus on these is low end power and torque, so this is not surprising.
What we want for our cars is different than what OEMs want now. For mileage you would like an engine that only ever turns less than 2000 RPM and just boosts for it's power. Why? Engineers compare engines based on the pressure a compressor would have to supply to the engine to make the same amount of power. You might have an engine that produces a peak of 150 BMEP, Brake Mean Effective Pressure. In town though and at speed limits maybe it produces 30 BMEP. If it's a V8 and loafing, what is the BMEP during the intake cycle when there is manifold vacuum? Perhaps it would be -10 BMEP. To make the 30 BMEP, it actually must produce 40 BMEP. The turbo might be at 0 boost, which is actually +10 PSI compared to the vacuum in the V8.
Pushrod V8's in the 200-300 HP range are good choices for us. A Ford 302 ( originally 221 ) are very small, mine is something like 21"-22" from bottom of oil pan to top of intake manifold. It has an aftermarket pan and manifold, but not expensive parts, just standard stuff. Aluminum heads and flywheel are not expensive for these motors and the whole thing is way less expensive than these turbo crate engines. You could even get an aluminum block and still be cheaper than the modern stuff. The sales volume on those parts is huge...
Other than that the smaller 4 cylinder engines like Miata and Focus make sense... For most people automotive parts work best.