Searching for similar devices only yields NSVJ3910 (again from ONSemi with a similarly braindead datasheet) and no expected 2N3xxx equivalent (the 2N3819 was a famous example, but I see no 2N3910), leaving 4 options:
(1) Guess. As it's an RF app, I'd guess 50 ohms, but confirm by option (2) or (3).
(2) Measure. Build an amplifier using one, and feed it from (a) a dead short and (b) different resistances. The resistance that increases the output by 3dB is the effective noise resistance of the FET. From that, computing the noise figure for any given source impedance is straightforward.
(3) Ask the supplier's FAE for clarification. On a £0.10 part I'll be highly impressed if you get a competent answer.
(4) Choose a different part, with a better datasheet. Motorola datasheets often struck me as weirdly incomplete next to National Semiconductor or Philips, and On Semi seem to be continuing the tradition... Look at 2N3819 or J309 (or SOT23 equivalents) or 2N5459 if low noise is a priority. Datasheets that show schematics of the test circuit used are usually a good sign...