If you're going to ride it, the foam will break sooner than later and crumble everywhere.
If you're not going to ride it and can live with the broken down external appearance, that is good.
If you cannot live with the broken down appearance, replacement of the old foam will have to be addressed.
Original old foam was not one solid piece of foam. Original foam was made up of thin, tall blocks, the blocks glued together with open (air) spaces between blocks.
With the seat flipped up, one can see the blocks and open (air) spaces.
If originality of appearance is required, then the blocks must be reproduced. If restored to look original, one solid piece of foam is not acceptable.
Yamiya makes one piece original type red foam, but not blocks.
Perhaps red foam could be found and blocks cut and glued like original. Then make the original contour of seat using layers of red foam padding on top of the blocks.
In order to do this, one would need an original example.
And, to confound, we know all seats' foam are not created equal. There are different contours between early and late sandcast seats. It is safe to say, there is a difference between shapes of the foam blocks ? I believe all sandcast seats used red foam only. Anyone know different ?
Perhaps "K0" foam is cut different, using layers of foam and fewer blocks and using white and red foam.
Many questions i do not have the answer for.