If you are going to hack up the muffler, we have no way of predicting what the resulting back pressure change is going to be. You are re-engineering the system to custom specs. You will have to either dyno the bike to get a proper fuel map, or test and try different jet sizes 'til it runs properly, and show nice color on the spark plug deposits.
I already use the UNI NU-4055 foam filter in my F bikes with the stock muffler, and no changes to the stock jetting. It's fine here near sea level. But, if I ever have to change out the stock muffler, I'll have to also change the 069a jetting for certain. An open exhaust installed on the F behaves far more like the free flowing, low pressure 4 into 4 that was found on the 74-76 K models. So, that jetting is a good place to start. (100 Main and slide needle clipped in the 4th position).
If you are expecting the stock engine to make a noticable difference in power with only an open exhaust change, you're in for a disappointment, imo.
You will lose some low and midrange power, for a very small gain around and above red line RPM. That is what a 4 into one normally does. The Honda stock F muffler operates at a higher pressure, in part to make street riding power more balanced across the throttle range. Your engine won't last as long at sustained higher revs, if that is of any concern to you. But, that is the sacrifice racers usually bear, along with frequent engine rebuilds. You will also need to downshift more frequently to get the engine into that upper power band.
If your main interest is in a canyon carver, the newer bikes do that far better than a 40 year old chassis and engine design, even with "upgrades".
How did you decide the old needles are "worn out"? I have yet to wear out any of them on my many bikes. I've got more than 50 of them here more than 30 years old. If they don't spring, then soak them in carb cleaner to soften the gum and flush them out.
