Actually the Thai BMW style pipes are not that bad. You will notice that the Thai exhaust pipes at the head are slightly different. You will notice that the original pipes have a smooth bend and the Thai pipes have a slight fold. The fold makes the exhaust flanges hang up at the fold. So you need to dremmel the opening of the flange and lubricate the pipe to avoid scratching your new chrome.
I suggest using a light application of fiberglass matting at the back of the damaged metal. As soon as the fiberglass is rigid and tacky, mix 1/3 fiberglass resin and 2/3 body filler together. Add both hardeners at their appropriate measures for their volume. Mix this resin/filler mixture in a Dixie cup. Within 3 minutes, pour the mixture in the voids to bond deeply with the glass and metal. Allow to cure 24hrs. Finish the body work as normal and paint. This is my secret to repairing damaged metal. The example below shows major deep rust damage.
After my resin/filler technique followed by standard body finishing.
