Uwe,
When I purchased my 1980 C30, it had what was called the 5411 atomic universal diesel engine. 10.75 HP @ max RPM (at the flywheel). Run down your shaft to the prop with the drop in horse power, and you were lucky to be pulling 6HP @ 2200RPM.
My boat couldn't get out of its own way. I looked at various models. I liked the Yanmars, but they were too high. Wanted to get the 25XP but I would need to cut and build out the front area in my settee. I purchased the 3M20A.
I designed transitioned brackets from 1/2" SS plate and they worked fine with my stringers. I have the drawings if interested. The Perkins engines were cheaper but once again too high profile.
On the older model 3M20A's like I have, I needed to have a 4 pass heat exchanger made because Florida water gets warm and driving thru passes would run hot. I do believe the heat exchanger has been upgraded by now.
Stu is correct in price, as you will also want to purchase the admiral control panel & wiring harness at the same time. You may also want to install it above where it is now to the backrest area cockpit seat. It's an easy install and just use a close-off plate for the old location.
As for the old enine, I changed out some seals, sandblasted, painted and re-sold it at a boat fleamarket for $2000.00. You can offset some cost if this works for you.
Good luck,
CR