yeah, some nice rotary files should work well, when used with a die grinder...they make all sorts from straight to rounded to ball and if you were to use a ball or rounded type to start the hole (putting it at an angle) and work your way down by changing the angle closer to 90 as you get towards going thru...then start rounding out the hole. Just be aware these are not meant to "plunge"; meaning you only want to be cutting one side, so to speak, if that makes sense.
I agree with what was mentioned about though to start with a drill bit first though, then do the above procedure....being it's rounded, take a sharp, small punch and tap it against the runner, so your bit has somewhere to bite and not walk. Don't go overly fast on it and use oil/lube to keep the bit cooler and sharp.
Anyhow, when I did my porting on my manifold and turbine housing those things cut thru the metal pretty quick and once you get the hang of it, fairly accurate, although you'll want some other types like flappers to smooth it out when you're done (although this isn't super critical since you're welding and not porting/polishing).
