I'm in the process of replacing an exhaust system that is starting to rust, and am wondering, are the aluminium heat shields that bolt on to the exhaust pipe necessary? Should I transfer them from the old exhaust to the new exhaust? They are starting to look corroded/rusted.

If the header is factory cast iron then it is necessary as the heat from the headers will affect the surrounding components as well as the intake temperature. The HM headers I had were HP coated and even though the factory shields could fit on, I left them off.

If you are referring to the heat shields on the rest of the exhaust system from the headers onwards, they are best put on but after market ones never come with them and there is no easy way to transfer them from the old bits to the new.

I'd be wary of parking a Magna without heat shields on the exhaust in a paddock with tall dry grass in case you start a fire - your car would be the first to go up in flames, along with any others parked close - but both my Magnas now have aftermarket bits without heat shields under the car and I haven't had a problem. But I haven't parked in any paddocks yet. lol

Any heat shields within the engine bay are there to protect surrounding bits from radiant heat, so I'm guessing they are best left in place.