If the prop is left fully forward then the RPM cold be used as per a fixed pitch prop to get you home
Well, with the prop fully forward, you won't demand more power from the engine than it can safely produce, but you might not get home as expected. In some conditions, to prevent overspeeding the engine, you might not be able to demand enough power to maintain level flight. This would very certainly be the case in a twin operating on one engine.
Does this mean you should not continue your flight home? Of course not! Generally, with some conservatism with power management, you'd have no problem completing a normal flight with no harm to anything.
Beginning another flight in that condition would, as mentoned, be another matter. Intending to takeoff in an aircraft with a known defect, and not in accordance with an approved minimum equipment list, puts huge responsibility on the pilot making that decision. Things like a U/S nav light for day only flight don't get most people too excited, but a primary engine instrument is more important than that!
There are many worse instrument failures than a manifold pressure gauge reading wrong. (Like my passenger suddenly shreeking in pain after takeoff in a C185 amphib. When I looked over, his bare legs (wearing shorts that day) were covered in very hot, black engine oil, from the oil pressure line which had just burst, behind the panel, over his knees! Do I want to go back to floating, with an engine I know I cannot start again? ) Some instrument failures require an immediate change to the flight plan, MP gauge... not so much..