D7 17A under carrige aviability

Yes, undercarriage parts are still available, usually purchased after market.
As far as value, unless it's in exceptional condition these old dogs are getting to where they are worth little more than scrap price.
The 17A is a good tractor and worth saving if you don't mind investing some effort and funds.

Is the exhaust manifold any good? Most are rotted out by the turbo. They are not available anywhere. I have been looking for one for 5 years with no luck.

Best to convert the sprockets to bolt on segments if you can. Cost me a grand for weld on unused old stock sprockets here in aus.

These are very good machines and like OM said worth saving but track gear will be costly. I paid 9k for one in aus and it needs a lot of TLC. I paid too much but I believe they are one of Cats best and the turbo whistle is worth the money alone! It did come with a massive winch, logging canopy underbelly guard and spare tracks tho.

You might also consider welding up the sprockets as a short term fix - mine are but I will replace them in due course.

Given D8s appear to be going for give away prices in the US I would think a D7 17a would need to be in very good condition to pull more than 5kUS.

Totally different ball game over here tho.

Hopefully they will become collectable one day😊