They’re decent engines of good metallurgy. The only problem with a few of them are poorly made carburetors. Otherwise they last a long time. I put a piston/liner from a 10 gallon MDS .40 into a Fox .50 case and it still runs to this day with a little pinch left. I have a couple other MDS .21 engines too - all decent runners.
MVVS are made in The Chech Republic and are a different breed of engine. Good engines, just distinctly different from MDS which were made in Russia or Ukraine. I’m actually looking for a piston and liner for an MDS .21 rear exhaust Marine Diesel. I have a boat I’m going to put it into, but the piston fit is a little loose for good diesel operation. (EBay buy touted as nearly new. Looked it, but didn’t run like it)