For your given sample size and the observed effect size your power was <50%, no need for simulation. If that's a meaningful effect size, the study was under-powered.
If that is not the case and the confidence intervals exclude a meaningful effect size, then you have a pretty good argument that the study had provided a pretty clear answer.
What's the particular problem with simulating? Simulate your drop out/ censoring process, condition on the simulated drop out times, then either simulate straight from a negative binomial, or simulate as a Gamma-Poisson mixture (useful if you also have a baseline period and need to capture the within subject correlation for that). Then apply your analysis model to each simulated dataset.