I now have Tempus Fugit Creme de Noyaux and can say that subbing with amaretto will be noticeably different. Not meaning bad, but definitely different. One change I did make, after seeing the comments by several Discerning Drinkers regarding the closely related Lily Cocktail, was to replace Lillet Blanc by Cocchi Americano. This probably resulted in a more flavorful, bitter drink, which we both enjoyed. Don't know if old Etonians will agree, but we can recommend it.
still not seeing the noyaux available for sale anywhere here. Guess I'll just have to settle for the amaretto for the time being - oh the sacrifices ;)!