You got any pics of your dad's split ?
The plugs are a standard size but you DO need a fairly slimline socket. I got my from chavfords (part off their own tool range) and it works a treat. Do however also make sure you've got a decent extension bar that really grips the socket as the rocker cover gasket will grab hold of the socket as you withdraw it!!! I speak from experience at how frustrating it is once you've got a plug AND a socket rattling about at the bottom of that bl00dy tube