A 500-4 is a good little bike. They're somewhat collectible these days, and they were built like anvils so they were pretty reliable. Spare parts (either OEM or aftermarket) shouldn't be a big issue. Bikes have both an engine number and a chassis number; those two numbers may or may not match depending on the make, model and age of the bike (I don't know about Hondas but on my old Suzukis the numbers are different.) You may need to give both numbers to the DMV but that will depend on the state.
The simplest thing, if this guy's dad is still around and he is indeed the last registered owner, would be to have him fill out a lost title form at the DMV, get the replacement title, and then sell it to you. However, it sounds like that probably won't be an option if he's not being that helpful. The other choice, and this depends again on your state's DMV, is to just go ahead and buy it, and then apply for a new title yourself. They may or may not require you to post a bond while they check to make sure it's not stolen and generate a new title. For a motorcycle, the price of the bond shouldn't be a big deal.
Go to your DMV's website and look around, they most likely have information on what you need to do posted there.
