Your heat exchanger can be cleaned by an honest heating contractor. I’ve never heard of a hopelessly clogged ht. exchanger on a boiler.I would also keep the hot water system and not install an oil fired furnace . Get a second opinion from another contractor.There is more money in a replacement for a contractor than a repair . If your exchanger isn’t cracked and doesn’t leak it should be o.k. how efficient is your burner ,what percentage and how old is the burner ? burners can be replaced ,just the burner.