What's best engine to motorize a road bike

I have a road bike I use for commuting and I’m about to move 20km further out, so I’m considering fitting a tiny engine to it to assist my ride to work.

However, where I live I have a restriction – the maximum power allowed is 200 watts (about 1/4 HP).

I’ve seen some kits, but they have huge engines (beats me why they have to be so big).

Further, I’d prefer a diesel engine (no spark/glow plugs) and better economy.

Some brief research has shown that a tiny (5cc or so) diesel engine should be able to produce about 200 watts. A larger one could be de-tuned. There are model aircraft diesel engines of this size, but they’re quite expensive.

I’ve been told a "brushcutter motor" (small two-stroke) could be an option, but they are noisy, smoky and smelly.

I’m not worried about how to deliver the power to the wheel – I’m trying to pick the power plant first then make it work.

Has anyone got any suggestions for:

• diesel engine
• very small (200 watts max)
• not expensive

Edit:

My bike is a Focus Planet TR 2.0 (2012), which has internal hub gears, so the solution can’t involve replacing the rear hub. It also has disk brakes front and back, so replacing the front hub may be problematic (not sure).

Regarding range, I will live about 30km away from my CBD and batteries seems to have about a 60km range. Also, recharging takes a while, but filling a tiny tank takes seconds.

Also, I seriously doubt that the pollution created by a tiny 1cc engine exceeds that pollution created by the recharging process of burning coal and pushing the electricity through all the transformers and switches between the power station turbines and a bike battery.

For these reasons, I’m still seeking an internal combustion solution.