I suggest you stay well away from ANY type of silicon sealant because if there any any traces inside of the motor joints that pat of the silicon will NOT cure, instead it is just waiting for the engine oil to 'move' it around inside the motor where it will, not can, restrict oil flow with potential disaster consequences.
The solution that I have used on EVERY Vincent rebuild I have undertaken is Loctite 510 , It is an aerobic cure but better still, any uncured material will dissolve in oil so there is no chance of it causing any problems. I recommend an oil change after the first couple of hours of motor operation.
510 has added benefit of being able to seal gaps of up to 0.25mm and being low strength it makes any future disassembly easy.
Martyn