Both of these firms stock valves for your 15L (same as 14L). I spoken to both and ordered a complete set from Egge, they also stock stainless steel exhaust valves. Another member near me purchased valves for his 14L from Then & Now Automotive and was pleased.
You might be able to match them up with some tractor valves. Massey Harris and JI Case used Continental engines into the 50's, so did Kaiser Frazer, Jeep, etc. The old Continentals were used in Grahams, Reo's, etc. A machinist can modify them a little, and you can use oversize valve guides if needed.