If you just want to scale from 72 MHz to 36 MHz, then you'll have to change both the inductor and the wire.
Double the length of the wire, due to the wavelength being doubled.
This will get you to the same impedance at the feedpoint.
To tune the antenna, the loading coil reactance has to have the same value as the reactance of the antenna wire.
You have halved the frequency, so you need to double the inductance of the coil to get the same reactance.
Don't forget the wire pigtail on the receiver counts in the length of the antenna. So, at 72 MHz you have 6 in. of wire + 4 in. (say) of lead, giving 10 in. total. Double this to 20 in, then subtact your 4 in. lead. So, in this example, your 36 MHz antenna wire needs to be 16 in.
This is all pretty rough, since the shorter you make your antenna, the more critical the tuning. The best way is to get a bunch of inductors of different values, from say half to double the calculated value, and try them one at a time until you get the best range. I'd only bother with this if you wanted the best possible antenna, instead of one that's good enough to get the job done. If "good enough" is good enough, try the values calculated from the above, and if you get satisfactory range then you are done.