for starters - only go 2 teeth max and you must be using say a m115 spring
the best bit of stuff on cheapy side is a flexible dremmel shaft
or a kit like this:
http://www.ebay.co.uk/itm/Combitool-Rotary-Multitool-Combi-Multi-Tool-240v-Electric-Dremel-Compatible/131628193182?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140122125356%26meid%3D634a324eaa8a4db386b91fd02fb12e10%26pid%3D100005%26rk%3D4%26rkt%3D6%26mehot%3Dag%26sd%3D281944567721
you can hack the teeth off with hacksaw and file away the rest, yes you do need a file or a decent set of files for any tech stuff
but think carefully about shortstroking and go in stages 1 or 2 teeth max - you can remove a tooth later but can't put it back on
ask a local garage perhaps - though time you give them a drink you may be best off looking into getting some basic tools for work yourself
May I stress that shortstroking is for guns cycling very quickly on higher springs trying to reduce overspin - aprox 25rps+
if the rest of the box/gun is not assembled with a pretty high degree of care and attention it will not last long
it is not that hard but you need to be experienced in the rest of the box running ultra sweet before just trying to go faster n faster
you should already have a good little file set, san/emery paper, maybe digital vernier gauge for assisting in shimming and check stuff
be very fussy in getting piston light as possible blah blah blah and above all being prepared to spend days even weeks if needed to get parts needed/fitted as best as you can - a day rebuild is either pro stuff with everything to hand using tried n tested materials
or in most cases I believe a bit rushed if trying to tune that much
well that is my take but I never rush stuff - if you don't do stuff to the best of your ability then why bother is my usual thought
(of course if you are stressed or really pushed for time then yeah you gotta just say sod it at times but I still don't like it)