Need more info but this so far, What data rates(baud)? Gate capacitance of FET, Gate driver circuit, switching speeds on/off times, RS232 full level translation? full hardware control/ rx tx gnd? Mosfets are easy to come by with a lot of suppliers and choices.

Not fully understanding what you want to do.

P.S replies might take a while but will be made(have very little free time available)

An RS-232 driver will switch the FET's gate, turning on or off an LED. Assume maximum RS-232 baudrate. I just need to know whether a JFET is fine or if a MOSFET is necessary. I haven't shopped around for either but just picked one of each that will do the job.

J-FETs can be very useful devices, but I would choose a logic-level-gate MOSFET for this application. Drive circuits for these MOSFETs will usually be the easiest to implement.

At 115k baud you got a 8.7uS bit width. I would start to suggest a 2N3904/MMBT3904
transistor as Randall suggested simple and easy. You would obviously not be able to see the flash rate, so i assume it's a simple comms tester. Personally used it for a high speed CAN bus activity indicator and had no problems.

The gate capacitance of the FET will be your speed limiter.

You are describing an opto-isolated RS-232 link. This could be implemented easily with a couple of logic gates. There are numerous families of chips that will drive an optocoupler. Also some isolated driver/receivers.

What baud rate?

Do you have +/- supplies for proper 232 levels? Or will this be a 5V system?

What's wrong with an ordinary transistor?

Depends on how much current you can source. FETs don't sink much and switch quickly. But a good logic gate will do the trick so a transistor should work too.

All the suggestions were helpful, thank you very much!