Forgive my questions, but isn't this little Honda Engine 6 volt unit? Maybe the newer ones have been changed to 12 volt, but every one of the little Honda 90's I've had were 6 volt.
Here in the US we have gotten away from the 6 volt systems on most of our motorcycles and so, I'm thinking finding a high speed electric fan would be difficult for us. But, you being in England, may have a unit that will keep the little engine cool.
You're going to need to move a LOT of air to keep it cool inside the box. I'm pulling for you really, but I see it's going to be a challenge.
I'm probably one of the few guys here in the US that still has a working 6 volt only battery charger!!!
When I look at your frame, the 400 Burgman engine come to mind as a perfect alternative to the little Honda. But, I'm from the bigger motor, work less, type engine guy.
I use to have a catalog of the Piaggio Family of scooter and bike engines. They would have one that would fit your needs exactly.
Too bad they aren't certified for US Emissions.
Eventually I want to build a Streamliner like yours and would love to use your body design on mine. I like an enclosed body for weather protection. But I want to use a diesel power plant so I can run a belt drive air conditioner. Mainly because I live in Texas and it
does get hot here.
Plus it a long drive across our State. So being able to hold a max speed is important.
I've built a lot of bikes and still have a shop full of project bikes.
Almost ready to start my Diesel Tiger build. I have all the parts, just need time.
Interesting rear suspension. Is that a linked unit?