Crower's Six Stroke Engine

Using a single cylinder petrol engine would make sense, because

1. A small petrol engine is typically half the price (or even less) of a comparable diesel.

2. Parts are more widely available.

3. "Performance" parts (high compression pistons, camshafts, etc.) parts are available for some small petrol engines, because they are used to power racing go karts.

4. Crower's work has been with a petrol engine.

5. Petrol engines are less efficient and are more likely to benefit in a measurable way.

6. Sophisticated highly-modifiable fuel injection systems are available for small petrol engines. There has not been a market for similar systems for small diesel engines.

7. A single cylinder is challenge enough, and the dyno used to measure performance can be small.

I'd recommend a Honda GX390, a robust design for which parts are widely available.