As you know Mike, stripped plug ports are in my mind too, so I've been reading up on inserts. The time serts from wurth are the daddy because they have a neat tool that slides over the tap/ream while that's in place and cuts a recess for the lip on the insert, the other big plus it's the insert last 3 threads aren't cut full depth. As the insert is fitted the inserting tool cold forges these last 3 turns and deforms the outside of the insert, sealing it and securing it permanently in place. Nice design. However its of little use to me. As far as I can see the 038 cylinder has 4 or 4.5 threads through it, so about 6-7mm thick. The shortest timesert is 9.5mm. if correctly installed the last 3 threads would go right through and inside the cylinder where they would be unable to expand against anything to seal and secure the insert. Ie, it won't work. The 460 cylinder may be thicker though. For thin cylinders though, a solid insert installed with loctite red lock and seal is your best solution I think.
As for on or off saw.... Remove muffler, set piston to opening exhaust but not transfer ports, stuff an oil soaked rag in on top of the piston. USE COPIOUS AMOUNTS OF GREASE AS YOU REAM/TAP TO CUT THE NEW THREADS TO CATCH THE CHIPS ON THE REAM. remove oil rag, blow out, inspect and clean more if you see a problem but you're probably good to go. This is how is done on 4 strokes like cars and bikes without removing the head ( rag goes in through plug port)