They'll never make more money than in the application they were designed for.
And yes, those stolen in UK will be sold elsewhere (and vice versa) - where exactly is not very relevant.
Above all: there is zero need to reinvent the wheel, a list of stolen engines exists and is published from time to time. Unfortunately I cannot say who is the owner (some person or department within the Rotax Company, but who?) nor where to always find the latest version.