I do not know of anyone reproducing new valve springs,
I was lucky a few years ago and bought two NOS sets of springs for a 25 from Tom Underwood at the Portland Show, one set has just gone into my recent styled 25 tractor on this thread. With regard to 20-30 springs I have located a few for my projects from Gary Biewer at Barnesville MN, I would suggest giving him a call and see what he has lying out in the weeds.
Other than that it is the matter of finding comparison springs or taking an old one to a spring manufacturing company as a pattern to have a new set made, which I know has been done over here and expect you have the same facilities across the pond.
The same with gaskets, there has been a few NOS head and manifold gaskets coming out for sale on ebay and swap meets etc, but do not know of anyone doing full new gasket sets for the eearly Wallis and M-H tractors, again here in UK we have a good Company who reproduces gaskets in many different types of materials including the old style copper head gaskets with a modern replacement for the asbestos between the copper, they are expensive but still work the best for me.
Good luck with your projects and post us some pictures.