I haven't read all of the responses on this one, so forgive me if I repeat what someone else has already mentioned.
There are enough single-head multi-cylinder diesels out there to find a decent head, cylinder, piston, injector etc etc.
Scania, Volvo, MAN, Deutz and so on, all made engines with individual heads. The 14 litre Scania V8 is a typical starter for 10.
ALL early engine crankcases were fabricated, so don't get involved in big castings at all. A lot of modern-day engines had fabricated blocks, including the big Sulzer double-6 diesel, the Paxman engines and so on. There are no great mysteries there.
The expensive bits will be things like camshafts and gearing and enclosures for same.
Injection pumps are still being made.
Crankshaft has to be a fabricated job, but could also be turned out of a billet and heat-treated.
Oil pumps are no problem.
Flywheel ditto, can be turned out of a solid slice on a vertical borer or a wheel lathe.
Most of what is needed can be taken from existing designs.
Lastly, to see what can be done in the way of a fabricated single-cylinder diesel, look at the Caterpillar 1Y73 research engine:
http://www.oldengine.org/members/diesel/Caterpillar/CaterpillarMenu.htmThere are some castings in there, but it is still a pretty neat bit of fab work.
Peter