G'day Mate & Happy Easter down under. I just realized you're in the fall season, while here in the northern hemisphere spring is busting out all over.
I'm not at all savvy about gears and ratios. I wait for somebody smart to figure things like that out for me and go from there. Two who may know are msrfan and Quenton Guenther, both of whom know a lot about Whizzers and belt drive setups. I wonder if an adjustable front pulley would be helpful. If it were me and I was trying to get things lined up I guess I'd try to get the motor in place first, having it centered. Then I'd look at where a pulley can go on the motor shaft... is there much option how close in or far away to the engine it can be mounted? The third thing is the sheave. I would think it would need to be very close to the wheel since the bike frame may get in the way of the belt. I see on msrfan's builds that he has the frame crimped a bit where the belt would otherwise rub against the frame. On the other hand his 5 hsp Briggs engines are wide while your Villiers looks to be more narrow. His jackshaft arrangement (if that's what you call it), allows for some belt adjustment in and out from the frame, or at least that's my understanding. I imagine you're going to need to determine whether or not you need a jackshaft arrangement to get the belt going to the sheave in a straight line.
I shouldn't even speculate on this stuff, I know so little. The little book of msrfan's Briggs builds which I have made a copy of for you (hope that's OK msrfan) and will have in the mail tomorrow, should help to give you some ideas you can apply or adapt to the Villiers build.
I'm in the lucky position of having you to work all this out ahead of time so I can profit from your experience. By the time you are riding around I'll be starting on mine and pretty much "doing what Harry did". I'm afraid your pulley sizes may be partly trial and error. Are you figuring on some kind of automatic clutch or a manual clutch? I don't have much of a picture in my head of how you are going to go about it. Do you figure on pedal start? Gosh, so much to figure out...
SB