Merc crank plugs

I have a merc crank here that is out of a really gummed up engine. Shure needs the oil channels cleaned. Since the plugs are 5/8", what is a common plug to use. I can't buy these 5/8 plugs seperat over here. I might need to get a complete engine freeze plug set. What engine does have these plugs. Or how about tapping and screw in some plugs.
Any tips would be great.............

 

Red's headers sells 5/8" crank plugs.

Flatman

 

I tapped mine out using a 3/8 pipe tap and used allen head pipe plugs,of course i had the crank re balanced.A little more work but you don`t have to worry about the press in plugs coming out.