Raymond's on the right track, there is a serious problem in that engine somewhere, if oil is getting into the radiator. It needs to be located and assessed for options, but a replacement block is likely the route you need to go. Best of Luck.
'52 Cub ("Great Personality") 148xxx
'48 Cub with FH ("Gunny Cub") 38xxx
'57 Lambretta (a slow work in progress)
'74 Triumph TR6 (Mama's toy)