The Zodiac Mil-Pro website is currently recommending
80hp for an SR5.4, but it doesn't look like a maximum, just a recommendation.
As far as I know, only Suzuki and Honda make a 80hp motors, and they're both about the same weight as their 90s. The other manufacturers seem to prefer 75hp, and they all seem to be the same weight as their 90s as well.
As far as power–weight goes:
Yamaha 90AETOL 2-stroke: 90 ÷ 120 = 0.75 hp/kg
Yamaha F70: 70 ÷ 117 = 0.60 hp/kg
Yamaha F90: 90 ÷ 166 = 0.54 hp/kg
E-Tec 60: 60 ÷ 109 = 0.55 hp/kg (the nearest to the Yamaha two-stroke for weight)
E-Tec 90: 90 ÷ 145 = 0.62 hp/kg
So the 60hp E-Tec isn't really all that good for power/weight, but the 90 is the nearest thing to the Yamaha 2-stroke (25kg weight penalty), which you can't get new for leisure use nowadays anyway.
So if 90hp is an absolute 'must', the E-Tec 90 has it, but if the extra 25kg is too much, Whisper's suggestion of the F70 looks better.
Not that power and weight are everything, of course