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On my '77 KZ650C1, I replaced Michelin Macadam tires with Pirelli Sport Demons. The Macadams were fairly old at that point and had about 6,700 miles on them and they were the source of a high speed (90+ mph) wobble despite being properly mounted and balanced and having very much tread left. Below 90 mph the wobble was not noticeable, and even though I only hit 90+ occasionally the thought of the wobble waiting there for me was unnerving. I replaced my bike's wheel bearings, steering stem bearings, and swingarm bushings in trying to eliminate the wobble; I already had new top quality shocks. I'm glad I replaced those items, and doing so reduced (masked) the severity of the wobble a bit, but those items obviously were not the source of the problem as it persisted. After replacing the Michelins with the Pirellis the wobble was totally eliminated. I have taken the bike up to an indicated 120mph and it is completely stable. This problem was 11 years ago and no trace of the wobble has ever returned.

I'm not saying that your wobble is a problem with your tires; it may or may not be. I'm just letting you know what I experienced on my bike. One question - did the wobble start gradually over time with the current tires on the bike or did it start when the tires were first mounted? I ask because the wobble on my bike gradually started over time. I guess that was as the tires wore some or aged even though they looked like they were in very good condition. Ed

Here is all I find on front bearings. Bluej58 and 650ed have great suggestions also. Hope we armed you with enough info to solve your issue

40 MPH always seems to be a key speed in problems like this. As has been stated, if you can't find anything really wrong with the front tire/wheel don't overlook the rear tire/wheel. It isn't unusual for rear wheel vibrations to feel like the problem is in the front. I assume you have checked the front end for play or movement by pulling and pushing on the fork legs correct? There should be no movement at all when doing this. See picture C9 in Mikaw's attachment. Do you have tapered bearings in the steering head? I don't see any wheel weights on your front wheel. Does it have any? If not I would definitely consider that as a potential source of your problem meaning get your tires and wheels balanced and go from there. You can also examine or replace your wheel bearings at the same time you are getting them balanced. Unless you are reasonably sure the bearings are good I would go ahead and replace them as it is prime time to do so. Again, don't neglect the rear tire and wheel.
Rick H.

These are the lines I mentioned, they should run parallel to the rim and not fluctuate
Also look for waves in the side walls and weird deformations on the tread

Does the vibration disappear when you go faster than 40 mph? Or does it get worse?