Help with Papa Bear Chair Cushion  

If the original foam is all...
If the original foam is all crunchy dry, then it is definitely latex. I'm not sure they had a lot of different densities back then. It's not a thick cushion, right? I think a medium density would probably be pretty comfortable for most people.
I just got some firm and soft density latex, 3" thick, from foamsource.com and the soft doesn't feel drastically different from the firm, for what it's worth.
http://www.foamsource.com

Uniform
The density of the foam in your seat cushion would be uniform, not varied. As spanky said, you can simply replace it with a good quality medium to firm density foam. The cushion just gives an added measure of comfort, the webbing is what supports the sitter. If your chair is all original, you may what to also have the webbing tightened or replaced if it is sagging or deteriorating.

Spanky, Pegboard, thank you...
Spanky, Pegboard, thank you for the info. This will be easier than I thought!
Pegboard, as for the webbing, do you know how taught it should be? It is original and in good condition, but there is some slack to it. It's not incredibly loose, but when sitting there, is droops down a little bit. I thought I read/heard somewhere that it was made with a little droop so as to be more comfortable, but maybe I am imagining this. Any ideas how it was originally?
Thanks again!
Mike

Based on the cushion shape
and the foam contents, a good upholsterer with knowledge and experience doing good mid-century pieces should be able to replocate the foam insert....and possibily improve it with the newer foam that last longer.

order one from PP
seems a simple option as they make the chair today and will make / sell you the correct latex cushion.
http://www.pp.dk

Simon, I have contacted PP...
Simon, I have contacted PP directly and no response, and the local dealer is having little luck, too. Otherwise that would be an option. Though I am kind of afraid of what they might charge for a piece of foam.
Any ideas on the strapping question? I've been trying to find photos, but can't find any of a chair I know is in the right condition.

Mike
The straps should be straight across, not sagging or drooping. That tells you they have stretched with age and use. If the original straps are sound (not frayed, dry rotted, or otherwise compromised) you should be able to tighten them. If they are shot, it's no big deal to replace them, nor does it effect the value of your chair.

webbing
hi. I will send you some images of my pappa bear webbing. On all the chairs I have had the seat webbing is always loose as the curve of the seat cushion fits better.
You could try and source a latex factory and see if they will pour you a new cushion.
A sinlge cushion will cost.

Simon, thank you for the...
Simon, thank you for the info. I have sent you an email with photos of my chair strapping.
Mike