[curtis73]

Not exactly compact, but if you want cheap and reliable, the old Buick 3.0L 4-cylinder is super plentiful in marine applications and parts are widely available at the parts store. There isn't much different between most automotive and marine engines, but the 3.0 has proven crazy reliable in boats. Most are 120 hp at 5000 rpm, so at 3500 you're probably right on your 90 hp target. They're also built to run at wide open throttle and 5000 RPMs all day so they should be up to it. They don't readily come with EFI (except the new ones) but adapting a TBI from something like a 4.3 V6 should be a breeze.

The 2.2 GM engine is also a good idea for simplicity and parts operation, but they aren't what I'd call heavy duty.

Ford's Ecotec would make a nice one. VW's 1.9L TDI would work. Avoid the Quad 4. They have some head cracking issues that are worth it in a car, but the heavy use might accelerate the problem.

Do a search on Ebay for Hercules engines. They make some gas I-4s that are top notch for your application. Parts are cheap and available, but not at the local parts store.

Given your operational RPM range, make sure you choose one that has a supremely low-rpm torque peak. Operating at 1800 rpm and heavily loaded (more than 1/2 throttle) should only happen if you are at or above the torque peak at 1800. That's really hard to do with a small 4-banger. Loading an engine for work when its below its peak torque can cause overheating, excessive stress on internal components, and generally not a good idea. Most peak around 3500. That's why my first suggestion was the 3.0. Its torque peaks very low.

If you're just trying to get quick power for the prototype, don't worry about it, but in your final design make sure the engine will be at or above the peak torque point while under load.