There are a lot of choices out there. First you need to decide upon a budget in mind, & see what your funds permit. A rebuild of your existing engine is the most cost effective, but I have heard a lot of criticism of the YSM 12 being underpowered. I used to have an S 2, 28 footer with a tiny YSM 8 horse power motor. It was very tough & reliable, but on the small output side. But that boat only weighed 5,000 lbs, not 10k like the C 30!
I have heard good things about the Beta replacement diesel engines, & of course the venerable Universal 25xp diesels are a work horse. Universal is now owned by Westerbeke & the parts have gotten rather expensive. But every sailor has an opinion & I'm sure you will receive many. Good luck on your repower.