The first letter of the engine number will indicate the capacity. If you google JAP engine numbers you will find the relevant information. This is one link I found JAP engine number coding
You will need RTA (road traffic act) cover. You can get policies that will cover shows but not use on public highway. You do not need to tax or register the Wheel Horse. If any insurer says you do they are wrong as the showground/rally field is a ‘public place’ not a public highway. NFU and others will provide cover.
The carb jet is set by the screw at bottom of float chamber. As it ran OK initially I would leave this alone. Does fuel flood from carb? If so the float may have a hole in it causing it to sink and not close needle valve as it should. Engine manual should give details of carb.
