Yup, We are going to need the full story of that gun before we can help you any more !
LOL.
The plastic insert can be done with liquid plastic, Look on youtube, There are some good vids on making a front dovetail, Hopefully someone will come along and post a link.
As far as the heat that it saw, I figure that the front sight and the hogue mono grips melted off that gun may have seen over 600*, Yet I dont see that the steel has strawed from the heat which happens at 500*, But you easially could have removed it while refinishing it.... So before I would continue putting time into it, I also would have it checked out by someone way more familiar with metals than me...
The steel can see 600*, even more if evenly heated and quenched for a second to cool below 400*,
BUT if the gun got cherry red and cooled slowly, The temper of the steel is no longer there..... To me it looks to have cooled slowly...
Advice above is right.
Please have it checked!
Peter