Aye Ian Itsme,
I am a bit stumped on this one, as there are so many practical variables to consider, without knowing how the bike is set-up.
If the magneto and carburettor are OK, then I would pull the clutch in and turn the engine over on the kickstart. This should ensure that the clutch is freed off/no sticking clutch plates.
Not sure about this bit, but you need to make sure the carb has an adequate supply of fuel, so use the small "tickler" button above the float bowl, if it has one, to depress the float and float needle so as to fill the float chamber to overflowing.
Now set the ignition to "RETARD" and the air and fuel levers to just open, bring the piston slowly up to "Top Dead Centre" (TDC) using the kickstart, then ease it just past TDC. Release the kickstart back to the top of it's stroke. Now, with a swinging kick, kick downwards on the kickstart lever and you should at least get a splutter from the engine.
The ignition may need some advancing rather than being fully retarded and the air/fuel levers will need juggling to get a correct mixture to achieve smooth running and this is all before you engage gear, release the clutch and juggle all three levers so that the engine doesn't stall!
If it were me, I would be finding a local member of the Sunbeam Owners Club or the VMCC, who knows what they are about with flat tankers and actually rides one on runs or more frequently, to give instruction.
SteveL may be able to enlighten us further, from his experiences as a young bor, with his Father's Sunbeam outfit?
I hope that you are able to get a shot yourself and put a grin on your brothers face. There is "intelligence" and "practical nouse" and on this occasion "nouse" is required in abundance.
Good health, Bill