To answer the OP...
Create a mask of exactly what was on there. Weed out the lettering, stick it on exactly where it was before (this is crucial).
Expose in same conditions for same number of days/months/years as per previous lettering.
Remove mask.
You should now (theoretically) have a piece of material which looks consistent edge to edge.
I have never tried this method - as I too have found it easier to just buy some new perspex, but if I had to do it, this would be one way I would try...
who's got that much time? It could have been years.