I've done a couple mods for 120. You only need open up the feed side of the camera, leave the take up side alone. The drive pawls on the take up are too small to properly engage a 120 spool, so a 620 spool is needed, plus there is much less work involved since you won't have to grind out that side.
First, remove the back, and tape everything except the area to be modified (this includes the front, top and sides--dust goes everywhere). You must be careful not to touch the tension spring(chrome thingie) and test fit often until you have the bumps shaved down to fit a 120 spool. I use a spool loaded with two strips of backing paper to replicate a full roll of film. The bumps that need shaved are the two on the bottom beside the post that holds the spool, and the two on either side of the spring, also the two at the top that protrude down (where it looks like the spring should continue but doesn't). Do not mess with the two bumps at the top that the upper post is between, as they are needed to lock the spool in place. Sorry, I don't have any photos of any of the procedure to show. If you are interrested, I have two extra Tourist bodies (no lenses) you can have for postage, just in case you have an "oops".