With the Fox .35 being made here in the US, you would think that a standard #1 or #2 Phillips would be a good fit. The JIS system uses numbers also and I have those in #1.2,and 3. I think these ate technically philister head screws because of the tall head that they have?? All that meat gives a lot of support to the the cross and a proper fitting screw driver. What ever engine I'm working on, I just treat each one as a stand along entity and test fit a screwdriver before I apply any torque to it. I have some Kline multi tip screw drivers that I used at work that I use sometimes because they fit, and also Craftsmen brand, and a whole bunch of others that I have accumulated through the years, because there just seems to be so many different fits out there.
The JIS drivers tend to work very well on real Phillips head screws, because while they do not engage the entire slot, at least they use the outer ends of the slots, where the local force for a given torque is relatively small. Philips drivers to not work on JIS because of the opposite effect, the blades only touch the screws in the middle of the slot, requiring a lot of local force, which is why they strip out.
Phillips screws tend to strip using Phillips drivers because they are designed to "cam out", push the driver out of the slot as the torque goes up as a torque-limiting "feature". Unfortunately it also causes the center of the screw slots to strip because as it is camming out, it rides up until, again, you are only putting force on the center of the cross, where there is the most force and the least metal to oppose it.
Many times a "stripped" Phillips screw can be removed with a JIS driver because it will go out to the end of the slots. The Fox is definitely one of those cases, it is *much safer* to try to remove or install the Fox Phillips head screws with a JIS driver than a Phillips. This is true even of the specific driver recommended by Duke himself - a Proto #1. When I was running Foxes, I saw this note in one of his "Dukes Notes" ads, and went to the not-inconsiderable effort of getting this specific driver down to the Proto part #. Worked no better than the one I had been using and checking it now, it is obviously too small and too pointy.
As always - test fit them as noted above. A proper fit will have nearly no slop in any direction. Proper JIS drivers in JIS screws have *no* slop, literally none, and you cannot either turn or tilt the driver in the screw when it is properly engaged. It is FAR MORE SECURE than an Allen wrench in a socket cap screw.
Usually, the correct size driver will look comically too large, and you might have to find a good fitting driver, and then grind down the shaft to get it to fit in the head fins.
Brett