Does your capacitor look EXACTLY like the picture you posted? Or just similar?
I don't think it should matter if the capacitor has a metal or plastic case. The main thing, other than the capacitor's ratings, it that it has to fit wherever it needs to go. If there's a chance the case can cause a short circuit where it's located, maybe a plastic one would be better, but otherwise a metal cap should work.
The capacitance rating is 16 microfarads, abbreviated as 16 mfd or 16 µfd. Some motor capacitors actually contain two or three caps, which is why some will list multiple capacitances, e.g. '45/5 MFD'. The actual capacitance is probably fairly important, so stick with the ±5 percent tolerance.
The voltage rating, typically listed in ac volts, needs to be higher than the highest voltage the device is expected to handle. 350 VAC would seem to be a bit of overkill for your generator, except that the capacitor winding is isolated from the generator output so it might get pretty high. So I wouldn't try to skimp on the voltage.
The 40/70(85)/21 is the capacitor's climate rating. It can handle temperatures between -40C and +70C, and can handle the upper temperature for at least 21 days. I'm not sure, but I think the '(85)' means it can briefly handle 85C. I wouldn't worry to much about those numbers.
Note that motor capacitors come in both start and run types. The starting capacitors are smaller for a given capacitance, but can only operate for brief periods. For your purpose you need a motor run capacitor, or a capacitor intended specifically for a generator.
Here's one that looks like it will work, if the dimensions (1.57 inch diameter x 1.75 inch long) are OK. Note the slow shipping, though.
16 mfd 450V capacitor