There is more to it then that.
I have been thinking a LOT about this exact topic for quite some time now.
I figured two setups would be nice to have.
I would like to base one off of the GM 6.2 setup (indirect injection diesels already have a high compression ratio, stupid easy setup, and already have a low pressure setup for the injection.)
And a much smaller engine, like a small VW, or maybe older benz. Thinking here is that they would make EXCELLENT power plants for house hold sized generators.
Think of that, a 5k to 10k (or up to 30k really) that runs on ANYTHING!
The truly hard part is designing a new head. I don't know of ANY other head that is close enough to how our multi-fuel heads are in design.
If one doesn't know.
Stupid simple terms of how our multifuel engines work.
Fuel is SQUIRTED into the cup in the top of the piston. Note the cup even has a cut in it to help the fuel squirt in without atomizing any of the fuel, yet.
Second, and the hardest part, the head is designed so that all incoming air is swirling just right, so that when it enters the cylinder, it is swirling directly into the cup in the top of the piston. The swirling air is what atomizes the fuel, which is obviously a technical feat to do this close to top of the compression stroke.
The high compression ratio is required to ignite the fuel when it is being atomized in this form.
Newer diesels use higher fuel pressure, smaller holes in the injectors, etc. to help precisely atomize the fuel.
So, designing the pistons is easy. Copy what we already have. Then try and find an engine with a head design that closely matches that of our MFs as well.
Also, having an engine that already has a high compression ratio solves a lot of other issues.