Personally, I think 115 hp is plenty on a 20.
I run a 1983 115 Merc Inline 6 on a Seafari. I'm pleased with it performance wise. I use a Quicksilver Black Diamond 13-3/4 x 15 alum prop.
Out of the hole and on plane instantly and tops out at 36 mph @5500 rpm.
Idles around at just under 2mpg.
3500 rpm gets 2.3 mpg and 21mph
4250 rpm gets 2.5 mpg and 27 mph
5000 rpm gets 2.4 mpg and 31 mph
5500 rpm gets 2.1 mpg and 36 mph
My motor is only about 300 lbs but I have 2 group 27 batteries in the stern hatches. The Yamaha 115 is about 60 pounds lighter than the 140 Zuke. That can be significant on a 20 inch transom. A little less concern on the 25" transom. Also worth considering are the E-tec 90 (325 lbs) and Etec 115 and the 115 Optimax (both about 375 lbs).
Keep us posted. Must be a bummer having to make such a decision
