like an F, an E, a PA/FA or a FP45, an F59 or even a Genesis. . . then you get about 16" if you're working right to 1.6" (7.5" gauge) scale . . . if you're willing to fudge a bit, you could go to 1.8" scale and get close to 20" across a car-body unit.
There's a guy in the UK that apparently built and ran a a 1/8 scale Napier Deltic, (spark ignition, though, IIRC) so there's no reason that something like a 16-567 or a 251 couldn't be successful in 1.6" scale.
When I was thinking along those lines, I thought of using the liners, piston/rod and heads from a a bunch of .60 or .90 4 stroke glow engines as the power assembly donors. . . nitro fuel gets 'spensive, though, so I was thinking to run spark ignition and straight methanol instead, with maybe a bit of castor oil to keep the valves and rings happy. No black smoke, but (with a sufficiently heavy flywheel) you could get a reasonable facsimile of the sound of a 16-567 or an ALCo 251 or 244. As a second thought, I considered pairing up a couple of the 1/4 scale chevy 350 kits and hanging a heavy flywheel and a Northern Tool PTO genny off the end of them, and grabbing some 450W E-bay brushless scooter motors for the trucks.
Then I came across a Rover 1S60 turbo-shaft engine sitting next to a dumpster, complete except for 12V starter battery, nothing wrong that a good compressor wash didn't cure, still full of clean oil, with 15 hours on the clock. (silicon valley dumpster-diving *FTW!*) It even came with an integrated reduction gear set . . . . And it's 19" across the wheel case.
So now I'm trying to find a source of 24x small (6" cube), high-power (1.5kw) motors (Lenze makes some 320V brushless servos that are tempting) to make a 1.7" scale copy of UP 59 and 60 when they were lashed together and running on propane. (what good is it to have 45kW on tap if you can't apply it to the track?
It'd be just my luck to get the thing finished and have Train Mountain self-destruct from politics or tax problems.