I wouldn't. As Gus said, class J are fast - semiconductor fuses. And the current limiting doesn't matter - gen sets are pretty high impedance and generally will not produce much short circuit. The 100kw you list I'd guess between 400A - 1000A.
I'd use standard stuff - class K maybe? Fuses wouldn't be my first choice, just because of the dangers of single-phasing
If the time to trip is a concern, you will have to get the generator damage curve from the manufacturer and pick a fuse that opens inside of the damage curve. Or, as mentioned earlier, put in a generator protective relay and either shunt trip a CB, or shut down the engine - or both.
All of this depends on the application. If it is critical/high profile, add all the protection the job can afford. If not, don't worry about it.
One thing to keep in mind is the generator OCP does not particularly protect the generator - and the NEC recognizes that. Look at the wording of 445.12.A, "... protective means suitable for conditions of use." That tells me the NEC expects me to design the system so it is protected as far as my customer needs it to be.
Luckily for us most generator systems won't easily burn up from overload. The drivers (engines) are usually sized to where the the throttle is wide open at gen full load. If the gen load is increasedvery much, the driver runs out of horsepower and slows down - and the gen trips off-line.
Let us look at the example I gave: 100kw, 480V, 3ph, .8pf, 150FLA, OCP 200A
Now put a resistive load (pf = 1.0) on the gen and push the load up to just under 200A. The circuit breaker is not going to trip. The load on the gen is 199A x 480V x 1.732 = 165kw. So the driver mechanical output to the gen is 1.65X the design spec. So the spec driver output is 134hp. But at max overload, the driver must put out 221hp. The mfg may size the driver for a 15% overload, say 150hp. But generally not much more that that.
So where am I going with this: (for small systems <300kw)
If the customer buys a generator package, it comes with a generic protective system (minimum protective relaying and a CB) that generally won't let the customer overload the unit. And the driver is small enough that the customer can't seriously overload the gen for long time periods. And the package might even do what the customer wants.
If the customer purchases just the gen/driver, then it is up to the engineer to design the protective relaying. And that may well go beyond selecting an OCP and conductors.
cf