I don't like to squash another persons idea but I will ask you why you want to start with a heavy diesel engine designed for a car?
The challenges will be:
1. weight - signifigant amounts of weight (case, connecting rods, pistons, and cylinder heads)
2. thrust - most auto engines have a very small thrust surface that wears even with the small amount of thrust from a throw-out bearing with a clutch.
3. Additional accessories for cooling and reliable oiling.
4. A cam design that is profiled for the 2500 - 3000 rpm range instead of 4000+ range.
Adding a plate between the block and head will lower the compression but it will also eliminate any quench area that prevents detonation requiring a very low compression. This "space" is also bad for efficiency because it isn't swept and never gets the exhaust completely evacuated. That means it will dilute the incoming air/fuel charge, making less power.
You need a large diameter crank (3") for torque rigidity that is hollowed for weight reduction with very large thust surfaces or area at the ends for thrust bearings rather an using thrust washers. It must also have wider journals for wear resistance with positive, full time lubrication at both the main and rod bearings. Cooling oil to the piston tops and full pressure lubrication to the piston pins, cam bearings, lobes, rockers and lifters. While auto engines get full pressure lube to cam bearings the rest is left to intermittent or splash oiling.
This is all in consideration of a low RPM (3000 max RPM) engine made for direct drive of the prop from the crank. My opinion is that you need something close to two cubic inches per HP for reliability as well as the mods I listed above.