Thanks Chaz, what you say makes perfect sense and I'm being particularly dim!!!
I haven't actually built the bike yet, I've got most of the parts I need for a dry build but I'm waiting on a set of wheels. I've got quite a few projects on the go at the moment so I'm keeping this quite simple.
I'm just using a Chinese frame with separate fuel tank with an 80cc (66cc) engine, Monark II style forks, Brooks Flyer saddle, Sturmey Archer XL-FS front hub brake with an S1C coaster on the rear, black rims and cream Schwalbe Fat Frank tyres. It'll probably be similar to this Sportsman Flyer but in a board racer style: