The cottered cranks (properly installed) are as reliable as any, and I think they add a little panache to these old bikes. (They are just a little harder to remove and install, and the cotters always require a little custom filing to fit).
Alloy cranks do save some weight - but the improvements you'll get from substituting alloy wheel rims instead will be far greater.
Your bike is in such nice condition. The original leather saddle enhances the value, and since it appears that you still have the high flanged rear hub and all the braze-ons, it wouldn't take much to restore it.
IMO, It would be worth the effort because as you have already observed, it is a nice rider.