I have a 1993 Kenmore

Question

I have a 1993 Kenmore Gas Furnace. For past few weeks it's been turning on and off with a big flame very often (few minutes). Now there's a loud knock that shakes the room and we're afraid to turn it back on. A plumber who I trust and know a long time doesn't want to put any parts in it and he says we need a new furnace and gave me a reasonable price. He says he doesn't want to be responsible for the family if something should happen if he just replaces parts and says he'd be surprised if it lasts a month after putting parts in. I don't have an extended warranty and it's cold weather adding to the urgency. If I have a Sears man come over and he says he can fix it, I don't know how soon he can do that (time of essence!) and if it'll stay fixed. What should I do?