Hi Chris,
The J310 is a fet with a high gfs, the J310 is the plastic TO-92
version, the U310 is the metal TO-72 version.
The circuit should work with most fets with a high gfs, like
J308, J309, J310, half of J430 and J431 (these are dual fets),
and of course also all the U versions.
I got curious and built the damn thing, it didn't work, but
I got it working !
I increased the supply voltage to 30 volt and lowered the
source resistor to 3k3. It comes down to a drain current of
about 1 mA and a drain to source voltage of about 12 volt.
I tested ten J310's from Motorola and the output voltage of
the oscillator was between 8 Vpp and 16 Vpp.
I also tried a couple old Siliconix J310's and they worked
without changing the original schematic.
I also tested a couple of old Siliconix J300's and they looked
very similar to the Motorola J310's.
I even tested a couple of 2N4393's in a plastic TO-92 housing
and got them working too.
Give it another try and let us know how many went through the
ceiling.
on1aag.