How old is the engine? Sounds like time for a rebuild: new piston, ring, connecting rod, sleeve and bearings.

Cold weather will ofter bring out the worst in a worn engine, as the cooling and intake air are so cold the engine will run cooler, and parts inside never expand as much as they do in the summer. If internal clearances are borderline on a hot engine in the summer they will almost certainly be too loose in the winter.

A less daunting alternative may be first to check the valve train, make sure things are adjusted correctly and see if anything is sticking or broken.