Heat pump, supplemental gas boiler versus oil fire boiler

Two different questions. The first question is the heat pump. Newer heat pumps do remarkably well even in colder climates, and whatever you do on the second question I would at least investigate installing one — probably mini-split, but there are other options. Then the second question is… why are you considering switching to a gas or propane boiler from the existing boiler? Even at current fuel prices it will take quite some time to recover the capital investment of a new boiler, so I wouldn't recommend switching unless the old boiler is giving trouble. You certainly won't save by putting in a smaller boiler — that would be a real mistake, as you need what you have on the coldest days, when the heat pump will lose capacity.