Dear Martin,
as Iosephus has already written, you can find Mike Bishop's plan for the Newstead segmentata under:
http://www.armatura.connectfree.co.uk/jrmes/j1001.htm
However, the plans are from 2001, and there have been some important finds in the meantime.
In my opinion, the chest and shoulder part of the plans is still very useful, although Arik Greenberg has suggested a few minor alterations, e.g. a different angle for the leather strap that leads to some of the rivets of the shoulder plates (his advice is very useful, as exactly this strap is causing most of the trouble with the cuirasses of LEG III :lol: )
You can find Arik's article here:
http://www.legionsix.org/Newstead%20article.htm
You will have to 'translate' the measurements from inches into centimeters.
More information provided by Matthew Amt can be found here:
http://www.larp.com/legioxx/newstlor.html
Now the girdle section is the tricky thing, as the 2001 plans show a row of copper alloy cast tie loops on both ends of all the girdle plates (with the exception of the lowermost one). However, as Matt Amt has pointed out, new finds from Iza and Stillfried show that (most likely) only the left girdle plates had these loops, while all the girdle plates on the right side had a slot through which the loops of the left plates were pushed and fixated.
You should try to get a copy of:
-Bishop, M.C., 2002, Lorica Segmentata 1. A handbook of articulated Roman plate armour, Duns.
and
-Thomas, M.D., 2003, Lorica Segmentata 2. A catalogue of finds, Duns.
(maybe your local library can help you)
A final pice of advice: before you go to your armourer, study the plans, articles and find reports very thoroughly, and make a cardboard model as Iosephus suggests. This way you already know in advance what should be done and what should be avoided. And have a lot of patience