every little thing adds up, some tuners do restrict power big time.
look at mopeds today, most have a restricted exhaust, once removed they fly.

if the exhaust cannot get out the reverse flow causes huge power losses,
bigger carbs means more fuel, more fuel = more bang, but the big bang also needs to escape...
if you remove powerhead atleast you can see if any other restrictions are there like in the adaptor itself, plus clean it out of any salt or scale build up.
the powerhead is easy to remove and gasket is only cheap.
clean up the adaptor but dont go made in the exhaust area, smooth it out as much as possible to give nice flow.

instead of going to normal 50hp stuff, why not look into what the 50hp has done to it when in race trim ?
parts will probably cost the same or even be a touch cheaper to buy.

some of those 50hp Tohatsu / Nissan motors are bloody fast.

just a couple of thoughts