I like to restore things :) Dunno why - but I do.
And one of the things I enjoy restoring most is old engines.
I enjoy finding an old engine that's been rusting away in a paddock for the last 70 years - thrown away as junk over half a century ago. Getting the old thing home, doing the research, slowly disassembling it, then taking it piece by piece and restoring it back to as close as I can to the day it rolled off the factory line.
Cleaning, de-rusting, prepping, painting - it's a relaxing, cathartic process, good for the soul - and certainly good for the engine.
Then, piece by piece watching the old girl take shape again, until the great day when she first fires back into life after all those years.
It's not an expensive hobby - or rather it doesn't have to be an expensive hobby - like anything, I guess you can make it as expensive as you like. You might spend say $300 restoring a $100 engine - but then of course you can sell it if you choose. But you ain't gonna get rich selling restored engines. As I said, you might spend two or three hundred dollars restoring a $100 engine - so if you look at it like that - if it takes you $300 to restore an engine, and it takes you six months to restore it - you've really only spent just over $10 a week - which is a pretty cheap hobby really.
But forget the dollars, its the satisfaction of hearing the beast fire up for the first time in 50 years.