Well. I'm one of these returned flyers, after a break of 45 years. I've built four models so far, with more in the pipeline. My local club in Australia, the Ku-Ring-Gai Model Flyers club has been a great source of support and fellowship.
Of course, this time around there is eBay, where I spend far too much time scouring for engines.
I flew Foxes back in the day, and have secured a few. However, watching my fellow flyers at the club, I've soon been converted to the OS LA and FP fold. I got a blue 25 LA from someone on this forum, who kindly went to the extra trouble to send it to Australia. It powers a newly built Magician (a model I once flew).
I've been paying extra for NIB engines, partly because these are often the only ones to be had, but also because the performance is guaranteed.
I'm sure that there are many idle engines which could be repurposed, as you suggest. While I have enough engines (something I remind myself of every day), I will keep an eye out for anything you may find.
Cheers,
Tony