A few random thoughts on engines. As mentioned previously, with a plot like this, not much construction happens until an engine is decided on and sorted. About all that can happen until that point is sorting out some parts and more plotting. There's two logical choices, a bike engine or a car engine, and above all, something with a V configuration (that's not Japanese). So that's the basic parameters.
None of the engines I have here are suitable. With my own restrictions preventing the use of any old engine, sorting out a motor will be the highest cost component. A Harley engine would be nice, but fairly expensive. You could probably buy a unit Sportster motor for the price of just the bare motor in a big twin, but getting a cheap and practical reverse gear setup on a Sportster engine is problematic. A fairly simple job on the later big twin gearboxes, but a bit of money involved in buying an engine plus gearbox in reasonable condition.
Then there's car motors, the main issue being what will fit. You see some rat bikes with huge car engines that require a really high custom fuel tank, or fuel tanks located somewhere other than above the engine, bike style. I'd much prefer to have a more traditional style of bike tank over the motor. That brings up the issue of total height from ground clearance to where it will fit under a fuel tank. That makes it hard for carburettor motors. I think even a short block engine like a 253 would be getting too high with carburettor and air cleaner. Some people custom build tanks that have a big hollow to take that and even have intakes poking out the top of the tank. But I'd really like to use my 5 gallon Fat Bob tank rather than a custom. I've also figured out the Buick 3800 has too much bulk above the rocker boxes to practically fit.
One motor I like for a lot of reasons is the GM LS1 engine. For a 5.7 litre engine, it's very compact and only about 500mm wide. They have a cast ali sump pan that's not tall, as well as being strong. On the opposite end, the EFI intake is very low profile and saves a lot of height compared to most other comparable V8s or V6s. They are also light with an ali block and heads, and combined with their efficient water jacket design, you can get away with a smaller radiator than the iron V8s. A pushrod motor, so a cam upgrade is relatively cheap and easy. Wth the lifter guides, no need to remove heads and manifolds to swap cams. A strong motor with a skirted block, 6 bolt mains, and rods and crank in standard form are good to 700HP, so they're very popular with rodders and the like. Roller rockers as standard on a rocker cradle, beehive springs, separate valley plate, a coil for each plug and lots of other good design features. The stock exhaust manifolds are close to extractors in flow. As per the attached photo, the compactness of the engine can be seen.