I'm starting this for people who either don't want a kit engine, or who want to experiment with other engines. I've been looking around, inspired by mistaking a naked Bike Bug for a concrete /rescue cutter, at cutters like that. Most easily available engines are bulky along the like of the crankshaft, but some of the disc cutters aren't. This means a drive roller direct to a wheel can be added in place of the cutter, or a sprocket or pulley can be fitted. They're also usually above 50cc, powerful, and engineered for long reliable service under conditions of poor care or maintenance. My Mk2 may well get one of these. I also found there are petrol driven angle grinders with 90 degree bevel drive gearboxes. Although you'd have to work out your own drive from the disc mount, at least it can be a handy unit to experiment with in different locations. The down side, if it is one, is the low cc of the engine, but since they appear to have a standard 4 bolt mounting pattern, a bigger engine is possible.