F226 parts are widely available, just be carefule when ordering. It would be best to provide engine serial number. You can google Foley Engine. They have tons of flathead Continental parts. You can also try Jerry Moon at Montes Equipement in Chicago. Again, google is your friend.
With regards to "performance" parts, everything is going to be custom unless you want to stop with 0.060" over pistons.
You could bore much bigger, but it will also cost more money. You could install a performance cam grind (I can help). You could also look to stroke. I am sure that 3/4" would fit. The big question is: Is it all worth it? IMHO, that answer is NO. The best running F226 engine (at stock +20% over, mind you) is going to top out at around 75HP. With some mild massaging, you can get a near stock 44 to that for 1/4 of the money.
If you really want to pull a flathead Continental and the pullers in your area aren't that knowledgeable (many aren't), then you should look to purchase a B427 flathead, M330, or M363 Continental to drop in.
Perhaps Jerry at Montes could help you locate one. You will probably have about the same money in purchasing and installing the bigger flathead than working over a F226...