My point of reference is the old 21" walk behind mowers from the '60s that had 3.5 hp engines. Heavy grass over a broken septic line would bog them down terribly, but a normal weekly cutting was no problem. Add the extra horsepower necessary to move a 500 lb tractor and operator and 7 hp is (barely) enough for a 38" deck. For a 42" deck, 10 hp is more realistic for a gear drive and 12 hp for a hydro drive.How big is the deck? If it's only like 38" or maybe 42" and you take it slow and don't try to cut very tall grass it might work.