Just picked up a well-cared-for Green Thumb 5HP tiller and got manuals.
The address for Lawn Chief Manufacturing Company is:
201 E. Brink
P.O. Box 70
Harvard, IL 60033
The manual I have has a printing date of 10/85, so...
http://www.lawnchief.com/ brings up a True Value website, and I found a reference at Donald's Antique Rototillers (www.donaldantiquerototillers.com): "Lawn Chief was the store brand for True Value Hardware stores. Now part of the MTD family." The same site shows a reference for MTD:
Moll Tool and Die Products Inc.
Cleveland, Ohio 44136
Unfortunately, the MTD.com website is NOT Moll Tool & Die website, but only a personal website. :(
A yellow pages search on Lawn Chief and Moll Tool & Die both come up empty. :(
Best bet would be to call a local True Value store.
Then I'd try a local lawn tools repair shop.
Shouldn't be that hard to find a replacement for the seal cap. Looks fairly generic on mine.
Last gasp try... I did a "search by address" using the address for Lawn Chief and... and... YES! :D FOUND:
True Value Parts Central
201 East Brink Street, Harvard, IL 60033