Fixing the Hz gage:
The gage itself is an ammeter that is set for full scale reading at 200 micro amps. This can be tested by an electronics shop or anyone that can put the tiny 200 micro amp load on it. The gage is usually not the problem.
The transducer is the black box that is about 3x3x3 inches size located on the inside of the control box on the right side wall below the voltage reconnection switch. It can be checked and sometimes repaired. It has a (+) (-) and two a/c terminals on it. It is held in place by screws on the outside of the box. Once removed DO NOT undo any of the screws on the box as they hold the terminal strip on. Open the box by drilling out the 4 rivets, very easy to do. Then you may examine the circuit board for bad capacitors, usually the problem. The capacitors can be purchased at Radio Shack for 50 cents each. It takes 3 or 4. There are several variation of the circuit board. Just replace all capacitors with like kind. If the box has a plastic screw in it that is a plug to reach a POT screw to calibrate it so the meter is exactly 60Hz when given 60Hz input.
Why bother with all of this? Because that black transducer box is $150 to replace IF it can be found.
If the originality of the set is not an issue then Hardy Diesel (E-Bay) sells a combimnation Hz Volt meter for $20. It is rectangular and LED display so the panel would need a rectangular hole put in it and a surrounding faceplate to cover up parts of the existing gage hole.
With the generator a/c disconnected from the black box household 110 a/c can be put across the black box a/c terminals to see if the meter will work. Or put a meter on the a/c terminals of the black box when it is running to confirm that the box is getting input juice.
The Kill-A-Watt meter may be purchased for around $25 to $45 and used. This is a plug in meter that reads volts, amps, Hz, and PF. This is a good thing to have on hand anyway and will get you the hz readout right away. I got mine for $45 at Camping World but I think they are at Home Depot for less.
Those are your options. Good Luck!!