Looking at the Villiers Engine, I see what I believe is a centrifugal clutch mounted on the crankshaft, and then a fixed gear ratio of 3:1.
I am trying to visualize you getting the engine running and warmed up on the charcoal gasifier, probably with the trailer wheels lifted off the ground. Then you reduce the engine speed to an idle, get seated on the recumbent bike, and open the throttle on the Villiers 163cc engine which is supposed to accelerate with the weight of a concrete topped charcoal gasifier, the trailer plus hitch, the bike, and the weight of a person on the bike. Lacking a gearbox, I don’t see how this is going to get started. (I will be following your build carefully to see how this works out.)
I suggest using a torque converter such as this one, which only cost 70 USD:
One of my projects used a larger and more expensive Comet torque converter that could be built with different springs and cam followers that would change the drive clutch engagement RPM. The choices were 1600, 2000, 2200, 2400, 2600, and 3100 rpm. Once the clutch was engaged, the torque converter would change the pulley ratios as speed increased.
I am looking forward to more posts on your project. Keep up the good work!