  We explore the burst energy distribution of fast radio bursts (FRBs) in the
low-twist magnetar model of Wadiasingh and Timokhin (2019). Motivated by the
power-law fluence distributions of FRB 121102, we propose an elementary model
for the FRB luminosity function of individual repeaters with an inversion
protocol which directly relates the power-law distribution index of magnetar
short burst fluences to that for FRBs. The protocol indicates the FRB energy
scales virtually linearly with crust/field dislocation amplitude, if magnetar
short bursts prevail in the magnetoelastic regime. Charge starvation in the
magnetosphere during bursts (required in WT19) for individual repeaters implies
the predicted burst fluence distribution is narrow, $\lesssim 3$ decades for
yielding strains and oscillation frequencies feasible in magnetar crusts.
Requiring magnetic confinement and charge starvation, we obtain a death line
for FRBs which segregates magnetars from the normal pulsar population,
suggesting only the former will host recurrent FRBs. We convolve the burst
energy distribution for individual magnetars to define the distribution of
luminosities in evolved magnetar populations. The broken power-law luminosity
function's low energy character depends on the population model, while the high
energy index traces that of individual repeaters. Independent of the evolved
population, the broken power-law isotropic-equivalent energy/luminosity
function peaks at $\sim10^{37}-10^{40}$ erg with a low-energy cutoff at $\sim
10^{37}$ erg. Lastly, we consider the local fluence distribution of FRBs, and
find that it can constrain the subset of FRB-producing magnetar progenitors.
Our model suggests that improvements in sensitivity may reveal flattening of
the global FRB fluence distribution and saturation in FRB rates.
