I've researched this a bit further and found that the type of resin which binds the fibre glass is what dictates the temp it can withstand once set. Think I'm going to see if there is a suitable off the shelf resin that can cope with exhaust/turbo heat...failing that then its going to have to be the case of chop out the rot and make do with what is essentially a 20 year old shield but as original as they come! Thanks for the help!