Re: engine change out
The yanmar 3ym30 is the direct replacement for your 3qm30 as well as our 3gm20 on our C&C 35 MKIII 1983. Since the engine compartments are so small and almost custom on our boats I would say its best to go through with the Yanmar. It will involve very little modification on your engine bed. We have looked at replacing our also in the future. I would get an upgrade on the alternator if possible and get an Electromaxx.
The Beta is a great engine too and has been getting good reviews.