You will need drop outs which will take the hub gear axle.
Most derailer ones won't.
Filing is a bodge and may, or may not, work.
If the drop out needs fully remaking, it's a welding job.
Chain tension is something else to consider.
The existing drop outs certainly won't be sliding ones, so you may need a chain tensioner.
Chain line is another thing, chances are it will be OK, but you will have a very sticky problem if it isn't.
So doing the job to a decent bicycle engineering standard is far from simple, and could be time consuming and expensive.
Slapping on a hub gear wheel and hoping for the best could be done in 40 minutes.