Yes the engine is quite heavy Doc, - that cylinder barrel is cast iron afterall. Some engines have a beautifully fitted and finely machined cast iron piston too. Essentially the Villiers engine is a vintage design from the 1920s, the main bearings are long bronze sleeves with no seals and it's just the excellent machined fit of the shafts in those bronze bearings that keeps the gases in the crankcase. It is extremely rare to ever find a Villiers engine of this type with either a worn bore or sloppy main bearings, they are beautifully balanced and the materials used in their construction are top notch.
The downside of all that is of course that they are low reving engines and are not especially powerful, but they have excellent torque. Putting a tuned pipe on one of these Villiers engines is not really going to gain you very much 5-7HEAVEN because their porting and port design is very conservative indeed. I've owned Villiers powered motorcycles when I was younger, - Jemma would be familiar with 'Francis Barnetts', or 'Fanny Bees' as they were usually called. They felt very single cylinder 'thumper' to ride, not revvers at all and once the flywheel was wound up they'd cheerfully roll along on a wisp of throttle.