I worked in the Orvis shop in Tionesta for a couple years and prior to moving to the midwest, fished PA extensively for about 30 years. I grew up about 50 miles north of Tionesta in the NWPA diary country.
September is a good time to fish the area. The trout fishing in the region is fair, but not anywhere as good as the central or NC portions of the state.
Here are some reasonable trout destinations by stream size class:
Larger water: Oil Creek at Oil Creek State Park near Oil City, Pithole Creek in the couiple miles or so above its junctiion with the Allegheny. There are also trout all through the main stem of the Allegheny and if the water has started to cool off, you may find them off the mouths of any of the cold water tribs rising to late reason Isonychias. There won`t be many, but they can average about 20". Don`t bother with Tionsta Creek for trout this time of year. It is, however, an excellent smallmouth stream in its lower 30 miles, from Lynch to the dam.
Smaller water trout: The following tribs to Tionesta Creek - MInister Creek and the following tribs to the WB Tionesta - Two Mile, Four Mile and Six Mile Runs. Direct Allegheny River tribs - The upper half of East Hickory Creek, all of West Hickory Creek and in most years, Little Hickory Creek. Also, if you just want to catch a pile of little brookies and don`t care about size, you might fish Tubbs Run which enters the river just north of Tionesta along US 62. One fish in 20 will be over 7", but there is a pile of them there. Additionally, you might want to go down river a few miles and fish Hemlock and Porcupine Creeks near a little village called President on US 62. The access situation changes here from time to time, so you need to check that out. But these streams are 2 of only 10 or so freestones in PA with pops of wild rainbows. I`d also advise you to get a map of the Allegheny National Forest. Just about everything that flows has some wild brookies, some more and some less. It`s fun to explore.
Maps and local fishing info are available at the Orvis shop in Tionesta, Forest County Sports Center. I have no interest in same...

and really don`t know who is running it now. The guy who founded the business though, is a great, great guy named Tom. He might still be around if he isn`t out chasing his wife`s Brittanies at some field trial or another..
Good Luck..
If I were going there this time of year and could only target one species, I`d fish for smallmouth. Tionesta Creek is very good. Oil Creek from Titusville to the mouth is even better. The Clarion River in Cooks Forest State Park is a beautiful mountain river alive with smallmouth, mostly smaller fish, but some good ones too. And of course, there is the Allegheny. Probably the best bass fishing in the river is from Tionesta downstream for about 40 miles. In september, they will probably still be in the faster water hunting oxygen. The river bass can be spotty, but when you find them, you usually find a bunch of them.