Leo's are generally copies of the OS line .That said if it's old enough it's a copy of the OS25FSR or the OS25SX. There should be nothing unusual about these motors in their selection of plugs.If however you are using an FAI blend with no nitro then I've always favored the Enya #3 plug or any HOT rated plug. Some of the earlier Leos had some carb qualuty problems that gave you fits trying to find the leaks.
You might try Just Engines for any parts or further help. They have a WEB site and a Google search will find them.
Dennis