Putting the old Johnson on the Sceptre would be a good idea. I have a 1983 Merc 115 on a 20 Seafari and get 2.5mpg at 3500-4000 rpm cruise about 25mph. WOT 36 mph @ 2mpg.
The 115 Johnson would be a great match for the little Sceptre. Economy might be slightly better with the Johnson and a mph or 2 less top end.
I don't know about the manual trim issue. If someone is interested in the boat for what it is they'll worry about that later...Good luck.
