Sell as is
Retro fitting power trim twice would cost you the earth , I have just had to replace a power trim motor and quoted around £350 !
You have to refit the whole lot which is likely to be over £1k each for the units and little chance of finding second hand.
The best way to get the best price for the lowest outlay is simply a spanking good clean , touch of paint here or there , clean / polish of stainless , tidy up wires , fix that dial that hasnt worked recently , new strap on trailer winch etc .. etc...
Sell the fact that its a Humber ( great reputation ) & twin engines offers superb family safety & no expensive power trim to go wrong ! & that this boat would cost you £XXXX to buy new.
Dont the 55's have gas assisted struts so not a problem lifting engines anyway