Update. I bought a 26 inch Remo Powerstroke 3 Fiberskyn skin. Seeme timpany heads are measured a little different than other skins. Turned out the drum is not 26 inches, but 25. So now I have a small problem since I cannot fint drumheads (except timpani) in 25 inches.

I have therefore decided to go for calfskin (the drum originally had calfskin).

I guess the best and easiest way is to make a wooden ring to mount the calfskin on. I have no experience, but I am thinking to use a pine list, steam it and bend it aound the drum using cardboard to make a small gap (make the ring a little larger than the drum). I will cut it in an angel, glue it and maybe use some of those picture frame staples.

Will this work fine? Do you have a good source for calf skins?

you can go the calfskin route or you can custom order a 25" head from Remo.