For one of our disabled customers who want to use his trike for extremly long trips we built an AZUB T-Tris trike powered partly by pedals and partly by 4 stroke Honda engine. We had to do some changes on the frame, but all the parts around the engine are available on the market here in Czech. And we think also in other markets.
You can see the full solution on pictures incl. the Nuvinci hub and a Bar End shifter on the left side working as a accelerator.
It was an amazing experience for us, but we still prefer either a pure human power or electric kits like the BionX.