Paul,
I managed to completely replace the elastic on my old SE hood and it did a great job of resurrecting the roof for a couple of years. Prior to me doing it, the old elastic was sagging by a good 4+ inches (and that's man inches, not woman inches

) The elastic and even the bars would regularly come free and twat me on the back of the head as I drove

. Even when I got it replaced with the new S2 hood at Trim Tech, the replacement elastic was performing well and had not deteriorated.
From what I remember it only took one day of patience to do the job, with the help of my wife on some of the 4-handed jobs.
This is from memory so bare with me and ask questions if I've omitted details. I measured the width and approx length of the elastic required. Obviously buy more than you need to allow for mistakes, although the actual process is rather simple once you get going.
I went to the best local habedashery (?) shop and chatted up the lady behind the counter to enlist her help in sourcing the ideal substitute. They had some exact width elastic, but not quite the same tension strength, as the original.
You have to tackle the job with the hood open and lots of light pooring in, so ideally choose a good sunny day and do it on the dreiveway. (might have missed your chance there

) If you have a Dremmil style mini dril, that was the best option for drilling out the existing rivets. All of them are accessible, but the ones that rivet from behind the rear tension bar (behind and above your head) are the most challenging to access. You will have to take off all the furrry strips protecting the hood from the frame components, so either keep these in good order or try to source an alternative replacement.
Reattaching was straight forward, with the only challenging part being the reattachment of that strip behind the rear bar again. For me, it appeared impossible to get the rivets in from the back side of the bar, and therefore I opted to lap the elastic over the top and back round to the front side. Here I drilled new holes for rivets and rivetted from the front. It looked very neat and had the same tensioning characteristics as the original.
Make yourself very familiar with the original route / layout of the elastic before you preceed, so you know exactly which points are fixed and which are not. I believe I used some tailor's chalk to mark the points where I needed to fix the elastic. I put the hood in the closed position and tensioned the elastic as required. This needs to be quite tight and is where that extra pair of hands come in 'handy'

With the hood closed and the elastic being held under tension in the correct place, mark with your tailor's chalk where the edge of each of the frames comes on the elastic. Then, once you open the roof to provide you with the space to rivet the elastic in place, you can see exactly where it needs to be done to provide the perfect tension when closing again.
Ensure both elastic lengths are maintained the same throughout the process. Same lenghts when cut to start and tension both to the same lengths when refitting.
I am no mechanic or upholstery expert and this job was one of the first ventures into such an improvement. It prove straight forward to do with the help of a willing other. Go for it, take it slow and step-by-step and you should be fine.
Let me know if I can be of any more help and I'll try.
Matt