I have a Centaur & the previous owners replaced the engine with a reconditioned Watermota Sea Panther. I'm away at the moment so can't see the receipt, but from what I remember it was fairly reasonable and done at a yard in Gosport - fill in your bio and we'll know where you are. Remember you don't need a new engine provided the installers are reputable and you get a warranty.
Sea Panther is 28hp so it's a bit over-powered. It's quite a lump, not particularly economical but runs very smoothly & so far has performed excellently. Only real down side is the 28v required for the starter but later ones were 12v.
If you PM me an email address I can put you in touch with Ken Endean (writer of many articles in YM) who has had an outboard on his Sabre 27 for many years.
Bite the bullet and go for an inboard -recon or not. I put an outboard on my Hurley when the diesel died. Kept the boat for years. The outboard was ok but when I came to sell the boat the outboard was a real disadvantage. People expect Centaurs and boats of that size to have inboard diesels
Had a friend who had an outboard on a Leisure 22. Perhaps it was not fitted low enough down but the prop was out of the water more that in it. Look for a second hand non Volvo or look at fitting a Ford conversion. I had a Westerly Pentland with a Volvo which was drainng the purse to such an extent that I bought a second hand Ford 1.6 Diesel with the intent of fitting a marinising kit. I have seen a number of adverts for the 1.6 at about £75 down south. Kit will cost you about £1000 to £1500 depending on the gearbox. Cheaper if you hunt about for a second hand box, other most expensive item is the Water cooled manifold. If the Sea Panther which was a diesel version of the old 1.6 Cortina petrol engine then the "new" 1.6 should fit.
I had a friend who had a Leisure 23 with an outboard and the prop was out of the water more that it was in it. Fine for quite days but with a bilge keeler a good inboard is a sure way of getting off a lee shore.
I used to have Westerly Pentland and was so fed up feeding the Swedish Economy by the way of expensive spares for my Volvo that I bought a second hand Ford 1.6 with the intent of fitting a marinsing kit if the Volvo gave any more problems.
Perhaps you should consider likewise as one of the previous replies mentions the Watermota Sea Panther. This was a marine diesel version of the old Ford Cortina 1.6 petrol engine so if this fits the newer 1.6 diesel out of an old Fiesta or Orion should also fit.
I have seen 1.6 engine for sale at about £75 through Boat and Outboards etc and e bay. The kit to carry out the conversion will set you back about £1000 to £1250 depending on gearbox. Fitting an outboard will drop the value of your boat considerably.