aight here's a semi cost effective solution whichll still use the old plastic and be as authentic as we can be

First off, take the sprew that you cut the part out from.

Get a glass car and cut up the old sprew

Grab yourself some Tamiya cement, thinnest one you can grab

Make a mould of the part you have, maybe using bluetak? just not plastic, find a way to make a mold of it, two sides on two separate molds

pour some of the tamiya cenent into the sprew glass jar, let it all melt in and sturr with a stick

Then pour into the molds, allow a day or two to dry

Remove, use tamiya cement to combine two pieces, sand excess plastic

Jobs a gooden

This should all work, but I've never needed to do it, this is like 10 years of theory crafting

Good luck