A knock that gets progressively louder with time and mileage is probably a loose rod bearing. It would have been very simple to fix IF you had dealt with the problem early on. Now, you probably have a damaged crankshaft. I'd suggest rebuilding the knocking engine, even if it requires replacing the crankshaft, and having the rods machined for insert bearings while you've got it torn down. The only good reason to scrap the original engine would be if it's been bored WAY oversize, like .080" or more.
Jerry
