Haven't had any experience with this particular insert model, but in the past,
every Regency Gas appliance that I sold, installed or serviced was a high quality, high heat product.
They've been in business for decades (1979), so they must be doing something right.
You have continuity, but to you have COMPLETE sheathing?
That spark will go to the nearest ground & a break in the sheathing
(dry rot) will let the spark go ground BEFORE it gets to the intended location.
It can PROBABLY be done. Don't know of anyone who has done it, tho.
You don't say which model Regency stove you've got, but all the ones I've
serviced were KICK-ASS heating units & have lasted for years.
I think I'd sell it & buy a nice looking electric unit with the money...
The door should lift off the short hinge pins on the left side.
Lay it flat on a protected surface & spray all the glass retainer
screws with penetrating oil, like PBlaster & let it soak in.
Get yourself a hammer-type impact driver & carefully loosen the screws.
If they're STILL tight, spray...
I'd try brushing it with a stiff-bristle brush (NOT STEEL) to see what loosens up & falls off.
Then I'd try soaking a clean white rag in hydrogen pyroxide & holding the rag against the stone - DON'T rub -
on a spot off too one side to see if that has an effect. Once that spot dries try the brush...