I don't own an ICF-8650 but saw some on eBay. The listings that included the AC adapter indicated that the Sony AC-D468 is the correct power supply for this radio. There are eBay listings for just the AC adapter and I would recommend just getting one.
The AC-D468 adapter is switchable and the output can be set to 6, 9, or 12 volts. The radio needs 12 volts. The adapter current rating is 800mA. If substituting another power supply, higher power supply current ratings are ok. From a web search the outside diameter of the DC power plug is 5.5mm. I'm guessing that the center pin is 2.1mm although plugs are made with 2.5mm and 2.8mm pins. The center pin is negative which is a bit unusual. You want to get this right so the radio is not fed reverse voltage.
I wasn't able to find a web reference that said if the AC-D468 is regulated or not. It is old enough to be a linear supply rather than a switcher but it may or may not be regulated. Replacing the original supply with a switcher might result in electrical noise that the radio would pick up.
There are enough unknowns about the AC-D468 that buying some other power supply to replace it will require some educated guesses that may end up being wrong.