Ducati 250 twin was an option on the cars. Goes very well, but I have no idea where you get an engine, as it is not a bike unit, I think. Looks more like a marine or generator unit. The Lambretta unit works OK but its not really offering enough performance to prevent hassling from nurks who cannot overtake. Brits just are not trained to deal with puddle jumpers. The 250cc is normally governed to about 42 mph but would go faster. I think the Walkers freed theirs up, as it burned off most of the Microcars at Weston Park.
I am sure another engine would go in, but depends what you want to do with the vehicle. The reason they are governed is the, eer, handling. It does not really have much in standard form. So if you hot it up paint dice numbers on it and hope it is not a six!
Could always buy my Crayford Commutacar and have the top of the range version, which is British, of course. Hey ho.