Then it's either worn out, or something is wrong with it. It could be nothing more than worn out plugs, wires, have retarded timing, need a carb rebuild, etc. Do you maintain it, change the oil, or has it been run into the ground? Does it have a huge exhaust leak? Does it miss? You should barely be off idle to cruise at 70, even in a slant six.

Sure, you can put another motor in, but it would be a lot easier and cheaper to fix up what you got if it just needs some attention.