I don't know if you are aware the 021, 023, 025's all share the same chassis and there isn't much that doesn't interchange. I think I read here somewhere the 021 had a different stroke so the cases or crank shaft may not completely interchange .Maybe a part or 2 changed over years but I think they'll interchange. The ms210 is an updated version of the 021 and I think the engines interchange and a majority of the chassis. Ms230 and ms250 are updated 023 and 025's with the same interchangeabletype as the former 3.
Personally I highly regard the 025's for their power to weight and durability at home owners saws cost. The ms series I haven't used a lot but their changes I believe we're primarily to meet epa codes and didn't do any good to improve performance.