The info that you need is as follows:

I am not positive of the torque curve of the OM616, however, I am going to assume that it is flat between 3,500 and 4,000. If not flat, then the torque would be falling off a bit and the numbers below will be on the conservative side, which is preferable:

If the engine puts out 62 hp @ 4,000 rpm, it will put out 48 hp @ 3,100 rpm.

So, in theory, you could run it as low as 3,100 rpm and still get the power that you need for the generator. However, power figures being a rating for a new engine, it would be my recommendation to run the engine between 3,500 and 4,000 rpm to ensure that you have sufficient power available.

So, if the generator must run at a constant 540 rpm, you will require reduction gears of something between 6.48 and 7.40. A ratio of 6.48:1 will allow the OM616 to run at 3500 rpm and a ratio of 7.40 will allow the OM616 to run at 4,000 rpm.

I believe that it will have sufficient power at 3,500 rpm, and, for longevity, it would be preferable to run it there, if the engine is not too old and the power output is not degraded too badly. At 3,500 rpm, you would have a minimum of 54.25 hp available. You might have a little more if the torque curve is falling between 3,500 rpm and the data point at 4,000 rpm. This should provide you with enough margin to prevent cooking the engine prematurely.

So, your ideal gear ratio would be 6.48:1. Anything up to 7.40:1 would be acceptable. Note that there are no automotive gearboxes with a first gear that is that low. The typical first gear is about 3.5:1 or so.

You still have the option of running the engine slower and accepting less generator output. Nothing says that you must have full output from the generator if you do not need it. So, if you ran the OM616 at the torque peak (2400 rpm) you would have 44 hp available. If you did not want to fully load it at this speed (or if it is not brand new), you could assume that 38 hp would be useable. This still allows you a decent output from the generator; about 158 amps.

But, you still need a reduction gearbox with a 4.44:1 ratio to allow the engine to run at 2400 with the generator running at 540.

I would strongly advise against running the engine at less than 2400 rpm because the fall off in torque below 2400 will cause a massive drop in the available horsepower, and you would have a large generator that would not put out very much.