Quick & easy.

Select the group. Go into solo mode. ungroup it. Select the pieces you want to fiddle with. Group them (so you get a boolean group to work with). Select the rest of the pieces of the doodad. Group them (to make reselecting them later easier if there are more objects in the scene.)

Now you have two groups, one to work on, and everything else.

Select the group you want to work with. Exit solo mode. Enter solo mode. now you have just the object selected to worry about. Fiddle. Fiddle. Fiddle. Done? Ok.

Exit solo mode. select the two halves of your original group. Ungroup. Group.

Your original arch should nbe one group again. You might have to modify putting it back together depending on the fiddling you do.

It's quicker & easier to do than read.