I'm not sure understand the symtoms . . . You can turn the engine over with the switch, but it won't start? But if you push start it, it will run? If both of those statments are true, I'd put 99 to 1 odds on a bad ignition switch. The originals would last 25-35 years, but these cheap replacements that are out there now will last a year if you're lucky. The good news is that after you've changed it once or twice, it gets pretty easy to do. When I was driving a 74 Super and my wife was driving a 75 Super convertible as daily drivers, I could change the ignition switch blindfolded, in the dark with one hand tied behind my back.
OK, that may be a little bit of an exageration . . . it takes two hands . . .
If I understood you right, the oil light comes on at idle? That could be something as simple as the wrong grade of oil (but that usually isn't a big problem in cold weather) or the idle set too slow or something as major as the engine bearings worn out . . . or a bad sending unit, sticking pressure relief valves, etc.
On the power issue, it sounds like you my be running on 3 cylinders instead of 4. If the power dropped suddenly, look for something simple first, like maybe a plug wire came loose or the insulation rubbed through and grounded it out. It could be just in need of a good tune up. Have you adjusted the valves, replaced the points, set the timing, replaced the plugs, etc. since you've owned it?
