I've got a new rebuilt spare engine in my lockup. Trigger's broom has a new head!
You really need to tell the person in question there's some flaws in their desire.
For starters MZ engines aren't two a penny anymore like they once were. I mean, when was the last time you ever saw an MZ other than mine on the road? That means even the spares are all shagged out and anything you find WILL need completely rebuilt, no to mention kept going.
Also, two strokes like MZ engines do NOT like being used as tugboats. They NEED to have the wind blowing across their fins and they only run right when under a load, like going uphill or speeding down the road. At a leisurely (trike style) bimble for instance in heavy traffic they complain and splutter like annoying bastards and get miserable petrol mileage.
Also, an MZ engine is a heavy bastard for only 20-odd horsepower. Thank fuck it sits in a frame that weighs fuck all. There's loads of good four stroke engines that would produce more power for the weight and be WAY more suitable for tugging a three wheeler around. Pretty much ANY four stroke 250 would be FAR more suitable for the job.