Hartley oscillator and feedback fraction
if we increase the value of L2? (similar to Colpitts oscillator where we by changing C2 capacitor)

Almost -- but you want to decrease the value of L2.  If you swapped the capacitors and inductors, then you would, indeed, decrease the value of the corresponding capacitor -- but inductive impedance goes up with increasing inductance, while capacitive impedance goes down with increasing capacitance.

Do we decrease the the feedback fraction and increase the amplification fraction if

It's more complicated than that.  For any semi-reasonable starting point you decrease the feedback.  But you don't necessarily increase the amplification -- you just increase the amplification that is needed.  Just willy-nilly cutting the value of L2 (or adjusting the ratio of L2/L1 while maintaining the same resonance frequency) will eventually leave you with a dead lump of circuit that rings a bit when lightning strikes nearby but never breaks into continuous oscillation.  Before that happens, you'll have an unreliable oscillator, or one with poor operating qualities.