I've done hatches on my boat with the west system it really isn't difficult. Worst part is getting the old wood core out if it's not fully saturated and still glued together. Just remove hatch, cut out the wood from the underside. Clean it up, cut a new piece and then follow the directions on the west systems to glue it back in then add some fiberglass mat over the repair
V8 350 burns about a gallon an hour trolling. So 8 gallons verses 2 gallons on an 8 hour day. So to me it takes a long time to recover the initial kicker cost. Where the savings would be is wear and tear on the main engine.
Couple of boats on Lake Michigan are running large kickers. There is a 35 ft bertram with a 125 hp four stroke kicker. And also a 38 ft tiara running a 90 hp suzuki four stroke.