Pull off the head and see how many shims it has or measure them. Stock has 2 aluminum 0.30mm shims with a 0.10mm copper shim in between equaling 0.70mm. If it has 4 gallons on it, it need have low compression. You can usually get them to run, but it will only hold idle for 10-15 seconds unless you keep revving it. The stock glowplug to use is a Novarossi #6, in standard or turbo depending on the installed head button. The O'Donnell's 99 glowplug is another option. Being it is used, make sure it does have good compression though, as you will be chasing your tail trying to tune a worn out engine. Weak compression will eat glowplugs and so will a leaky front bearing or case air leaks.