Richard,
Some smaller airports stock "mo-gas" as automotive gas is called. It really saves some money over 100ll. You have to remember that most of these old engines were designed in the 1950's (mine in the 1930's) and have 7.2 compression ratio's. 80 octane was the norm until it disappeared 2-4 years ago. Many guys have their own fuel tanks at the hangar or just bring a couple of 5 gallon jugs and pour your own. Its not a big deal.
You have to adhere to TBO only when the airplane is used for charter or if your insurance demands it, which rarely happens. If the engine is running fine then keep flying it. The key to long engine life is changing oil often and keeping the air filter clean. Doing an oil sample from time to time to check helps as well.
I can show you a good Citab/Aeronca Champ forum. Funny thing is that I am the SYSOP there!
No idea on C-172 forums.
Site is:
http://www.aeroncapilots.com/NewForum.htm Over 600 owners and members and almost 10,000 posts there, so lots of information on the planes. You want to go to the 5th catagory down, the "Post war-Champ" section. Its the plane that the started what is now the Citabria today.
Believe that there is a Citabria forum, but need to find it. The one above will get you started if you are interested in a taildragger.