... got overtaken by an e-bike on Williamstown Rd in the car and I was doing 70...
It's 20 plus years since my bicycle shop days, when I fitted 25cc Sachs kits to pushies, so stretching the memory again. What I can recall is that to exempt the rider and motorised bike from motorcycle licence and rego requirements, the engine output was to be 200W, or less, and the bike was to be restricted to a max. speed of 25km/h - on the flat.
The specs on the Sach were spec'd to suit.
A little tuning and tweaking for an extra few W or km/h would / could be easily overlooked by the authorities, by 70km/h plus, would attract attention, and spoil it for the rest.
I may have told this story before:
Old Kev purchased a Sachs converted Malvern Star from the previous bike shop owner (Pete), it didn't live-up to Pete's claims of 25km/h and 100km/litre. Kev couldn't be convinced that he (at around 100kg), his tools in one pannier bag, six pack of beers in the other, was expecting too much from 25cc. But Pete said ...
So, I fitted to 1.50 x 26 high pressure MTB road tyres, pumped them to around 80psi, and fitted a Cateye cycle computer. Told Kev to come back every couple of days and I would top-up the 2 litre tank, and we would both keep track of the km travelled and fuel used.
A week or so later, after running through the figures, Kev was satisfied with the fuel consumption (around 100km/litre), but best of all he thought that I was a real champ, as the bike could now do 35km/h.
It was a WIN WIN result, no point in telling him that I fiddled with the computer calibration.