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No, the 1985 20 and 30hp are the same displacement! They are both based on the 31.5 cui block. The 20hp has a restrictor cast in the intake manifold and uses a carb with a tiny venturi. As far as I can tell, those are the only significant differences.
If possible, find an intake off a 30/35hp so you can "undo" any changes later...
I think you need a different throttle cam plate too..and a couple other little things. Find a parts diagram book for that year ...the ones I have covers the 20,30 and 35 hp all in one pamphlet and you look for different part numbers.
You will need carb, intake manifold, reeds, cylinderhead, midsection and lowerunit. The 35 has thru hub exhaust and better exhaust tuning in mid-section. It can be done, but best to get a burned 35 HP and us your powerhead.
The 1985 20 and 30 have the same lower unit - both through hub. The 20 has lighter duty clamp brackets, though, and may or may not not stand up to the stress of 30 horses.
