I had a Petter Mini Twin 12 in my Dockrell. It went well, albeit a bit of a rattler at low revs. I had no problems with it during the time I had the boat. It was a bit seized when I first bought the boat, due to water getting back into the engine through not having a water lock. When stripped, cleaned and rebuilt, it gave no problems.
As you probably know, there is a cold start arrangment whereby you remove a small plug and inject a small quantity of oil. This, by going in the top of the cylinder, prevents loss of compression due to "dry-ish cylinder walls - thereby increasing compression for cold starting.
I also had a Colvic 26 Sailer (Samara) for almost 8 years, and in that time went from Falmouth to the Channel Islands twice - via Salcombe once and Dartmouth once - so I know its capabilities.
Many years ago I had a Macwester 26 which had been fitted with a Petter mini twin the year before. I agree with a previous comment It was rarther noisy. As for reliability I supose it resonable. We had a couple of problems. Once the cylinder head gasket blew. It was at this time we found out that the engine would not run on one cylinder even though it was suposed to. the gases blowing passed the gasket had erroded a hollow in the top of the block. This occurred in the middle of the sailing season, so we effected a temporary repair by filling the hollow with epoxy resin and then polishing the spot back flush with surface of the block. At the time we thought we have nothing to lose as we expected to have to replace the block any way, as it happened the repair lasted right through the summer so we left it alone. We tried to run the boat on cheap car batteries, needless to say we often had to help the engine start with hand and batteries to get it started. On one occasion there was some slight rust on the shaft and when the engine started we could not disengage the hand start crank so I almost had a heart attack until we managed to stop the engine without the lever coming of and exiting through the bottom of the boat. We were pleased to have the hand start facility but we always kept it spotlessly clean and greased after that incident.
I owned a Colvic 26 for a number of years - depending on the layout chosen by whoever built it a wonderfull boat. Prefers sailing at a low angle of heel (upright ish). I had sacrificial wooden shoes glassed onto the bottom of the bilge keels as she dried out every tide. This revealed a problem that I have heard others having - when they drilled into the keels lots of water came out of the one!!!! There was no sign of any problem with the fibre glass that covered the ballast on the inside so how it got there I will never know. I had noticed that she heeled more on one tack than the other prior to this being discovered but put had put it down to my imagination. Even if you do not have sacrificial wooden strips put on if you find here heeling differently it may be worth your while drilling a "SMALL" hole in the keels - in fact - and others may disagree - it may be worth while drilling anyway.
I always felt safe sailing her and my sailing area was the Bristol Channel North and South down to the Scilly Isles. I dis remember the number of times I went past Hartland Point and never felt any concern for the safety of the boat.
Have had a Colvic 26 for the last 7 years.A very solidly constructed hull,and very suited to drying out especially with the vertical bilge keels.As there is no stress as you get on say the bolted on splayed out keels of Centaurs and Konsorts.
A very safe deepsided cockpit. Mine is an Atlanta layout and is professionally fitted out.There is a lot of variation between boats of the quality of fitting out as a lot were bought as bare hulls for home completion. But it is in my opinion a first class boat.not the fastest but you will get there. I have a Yanmar YSM12 single cylinder diesel which i have found extremely reliable. A friend fitted out another Colvic 26 and used a PetterminiTwin which he found very reliable.