Seals for a Plainsman .175 BB pistol?

My father-in-law wants me to fix his Plainsman .175 BB pistol.

It needs a new Co2 seal for the cartridge.

Also – Do I need to find lead shot for this or can I safely use .177 BBs since they are a bit smaller than .177 pellets?

Any other seals that may need replacement while I am at it? I tried looking on the Internet for info, but can not find any other than a reference to an old magazine article about resealing this gun, but I can not find the article online.

A diagram of the gun would be nice too.

This gun is in excellent shape otherwise. Looks factory new but for a few small scratches. I see that the 8 gram Co2 cartidges may be available at Ace Hardware or Restaurant supplies for fresh whipped cream canisters. And I found them online at an airgun sales site and also for a good price at restaurant supply websites in boxes of 50!!