Ouch that's a bit terminal.......
If you are on a budget, might be worth giving DA in Tayport a call - He may have a working powerhead attatched to a gearbox that met a rock that you could bolt on? (that was what I was going to do with my old Yam before I found the Merc) You may even find that the 40 & 50 shared a block, and can up- power at the same time. Can't comment on the ease of that not knowing make / model of the engine.
If it's a Suz or a Honda it might be worth calling Ardoran marine (nr Oban) He also has a graveyard, but biased more to Suz / Honda.