I’m in the process of resolving a faulty speedo, along the way I found a seized gear within the gear unit assembly. I have it 90% disassembled but the final gear within will not budge!
I’ve bathed it for hours in WD specialist penetrant and still managed to break the tip off of a screwdriver trying to free it.
ANY suggestions?

If you are talking about the small gear with the spiral teeth, then I've had a problem with that too.
I found the spiral teeth were worn to where they bound on the larger gear.
Luckily we had a cycle junkyard in town and I bought one off a low mileage bike.
I know this doesn't help you, but just saying it might be easier to try to find another one. Good luck!

I've rebuilt just one of those speedo gear units with a badly seized main gear. This is the gear driven by the hub. It was very difficult to loosen it but I managed. WD40 and penetrating oil are good starts. Soaking in diesel fuel is also good. You can use a hair dry to heat the parts. An automatic prick punch with a dulled point can provide impacts to help break the running surfaces loose. Remove the snap ring, drive tang and washer first. Your goal is to get the gear slid off the shaft. From there you can use sandpaper to clean up the running surfaces.

If you can find a replacement, that's the easiest. But I think with persistence you can repair the old part. The worm gear (which drives the speedo cable) needs to turn freely as well. Good luck.

It is the worm gear that is stuck, I have everything else inside disassembled, if I could just break the worm loose I would be in business!

The main gear was a pain, lots of pinches, cuss words, and even more taps with the hammer to work it upward. But nowhere near as tough as the worm gear, I may have to source a replacement

Sorry to hear that. In my case the worm gear wasn't badly stuck. You may still have some luck using a heat gun. The idea is to expand the ID of the gear slightly and separate the frozen metal surfaces. Penetrating oil can then enter. As I recall the speedo cable has a male square end. If you can make a tool with that same profile that can be used to help spin the worm gear loose. File down an old hex key or something similar. Things I would try if it was in my shop. Nothing to lose at this point. If you break it, then it's time to source another part. The OEM part # is 34X-25190-01-00 GEAR UNIT ASSY