Might be a long shot...but does anyone know of someone selling a 1977+ DT100 seat foam or reproduction seat? Seems like there are tons of pre '77 gen seats for sale, but nothing for this gen. I can restore my seat pan and I've got a good reproduction cover that looks OEM as heck...but my foam is quite rotted, missing chunks, and disintegrating. Wondering if someone makes a reproduction foam or a full seat for these bikes? If not...guess I'll just have to try and sleep well at night knowing I've got some janky foam under my seat cover haha. Unfortunately there OEM foam is messed up in spots that would be quite hard for someone with my expertise to fix up...I can fill in chunks on the top...but places like the lower sides where the foam is very thin and tapered (and now completely missing) I've got no chance of doing. 

I have come across a couple of 77 on seats, on ebay uk that are located in Thailand.

A good upholstery shop can make you one and fit your cover

There is a decent  looking used one on eBay (not mine) for $35.00 looks to be in decent shape minus the cover but you said you have a new cover ready, although the foam may be in as bad shape as yours as it is 46+ years old like your old one?
under this heading.
1977 Yamaha DT100  Original Seat 

1977 Yamaha DT100 Original Seat | eBay

Thanks GR8UNCLEAL I really should pay more attention on how to post the eBay links and 3rd party imaging to become a more productive member on this forum.
I know both you and MarkT have posted “how to’s” many times already.

"Thanks GR8UNCLEAL I really should pay more attention on how to post the eBay links and 3rd party imaging to become a more productive member on this forum.
I know both you and MarkT have posted “how to’s” many times already."

Don't do yourself down, Lizeec - your posts are very much appreciated and respected.

I seem to be able to just copy and past a URL (the HTTPS............. bit) at the top of any site. However, I understand that this doesn't work for others.

I still haven't got into using third party image hosting services yet! 

Thank you for the replies everyone, got a bit sick and stopped checking the replies on here. Seems like that listing was ended anyway due to some error on the seller's end. ah well! Still trying to see if I can bring mine back to some kind of useable condition.

Take it to place that reupholsters car seats, and they can easily make a new foam and fit the cover for you. Far easier and probably cheaper, than ebay.

The other issue is also the seatpan being so rusty that the hooks that retain the cover are basically crumbling when I try to bend them. I suppose there is the option of rivets but I need to learn how people to do it without having the rivets show through on the outside of the seat.