I would be reluctant to use a power tool. Fork dropouts tend to be made out of a relatively soft carbon steel, so hand filing doesn't take long. So you don't mess up the alignment or have to fix a weird-looking slot, I'd recommend getting a non-tapered large chainsaw chain file. These are round files that cut rapidly but don't have a taper to them (which helps keep the dropout slot even). They are long enough that you can guide the file through one dropout while filing the other one. Cutting that much metal won't keep you busy for all that long -- 10 minutes at most should do it.