I'll give this a shot. The duraforce engine is designed with an above deck exhaust but the exhaust flow passes through a transfer pipe under the deck and up into the muffler. also, the engine mounting plate, looking at the parts viewer, looks different. I own both of these types of mower but never thought about a sweap.

Likely different cables / possibly hub.

As you said the engine is rated at 5hp on the Lawn-Boy site.... In the years since it was made in the early nineties there has probably been some repairs done. Either a different engine or recoil starter with the sticker you see on it now. Doesn't matter really. The "F" series engine wasn't changed much so the horsepower ratings on all brands were, and still are, just propaganda. I have several "F" engine mowers with different hp ratings and can tell very little diffference between them.

The two holes need to be kept open. The upper one allowed oil which collects at the rear of the crankcase to reach and lubricate the bushing which will now be a needle bearing when you install it. Keep the other hole open also. Make sure the needle bearing when installed is below these two holes. By boring the remaining section of bushing .020 oversize the crankshaft will no longer touch and oil can reach the needle bearing through this space. The bottom end of the needle bearing with the bearing number should face out and be flush with the crankcase seal shoulder. Always press needle bearings from the numbered side.