Nippon Piston Ring Co. Ltd. might be worth looking into.
They are the OEM supplier for piston rings since at least the 1980's. I tracked some impossible-to-find turbo engine piston rings back to them a couple months ago. (And they were 1/4 the price of the discontinued dealer parts!)http://www.npr.co.jp/english/products/p ... index.htmlhttp://www.npr.co.jp/english/products/p ... html#parts
There are two easily identifiable "Isuzu Genuine Parts" boxes in the middle of the little picture, though the names of the companies they supply are not listed.
Their website lists valve train parts among the things they make.http://www.npr.co.jp/english/products/v ... index.htmlhttp://www.npr.co.jp/english/products/v ... html#other
They show and mention valve guides. I don't see valve guide seals.
They do not have an online catalog on their website that I could find. Of the people I could get to answer the telephone at their US offices, the first spoke only Chinese, the second spoke Chinese and Japanese, and the English speaking guy was on vacation and never returned my call. I ended up tracking down the information through a retailer website that posted the piston ring specific catalog pages.
Their piston rings go back to G161A and G161B. I would be surprised if they did not also have the valve train parts also.