The oil pump chain is behind the water pump on the inside of the crank case. The chain runs from the balancer shaft to the oil/water pump shaft and drives the pump(s). It is horizontally oriented from front to center in the case.
If you are hearing noise from the top of the cylinder, chances are it is NOT the oil pump chain.
If the VLA's have lost their charge, the engine will have to be pulled but only the valve cover and the cam bearing top half will have to be removed once the cam chains are loosened. Recharging them involves obtaining some Kerosene and placing the adjustor in a small jar filled with the Kerosene and then using either a special tool or a simple punch to basically pump the adjuster until it is full of Kerosene and then replacing it. There are 4 per head IIRC. The Clymer's manual has the procedure for bleeding the adjusters.
It may also be that the cam chains have stretched beyond what the tensioners can take care of but that's rare.
If the chains have slipped, it could be that the valves are out of time and are smacking against the top of the piston. That should get louder as the engine revs though.
One other thing it could be, if the PO didn't know what lubricant was for, the cam bearings may have worn beyond tolerance and are causing the noise as the cams rotate. Old oil can cause this and the bearing surfaces are very soft in the head assembly.
The easiest diagnosis may just be to pull the engine and tear down the front cylinder and see what's going on in there. What ever it is, I doubt that you will be able to adjust anything that you haven't already tried without the engine pull and disassembly.
93 VN 750 "Ursula"
Moved R/R 08Sep06
R/R rewire 17Feb07
New R/R, Installed 14Jun08
New Stator installed 10 Jun08
Maintenance Free Battery 12Jan08