Ah yes, the Achilles heel of screwed tank bowls! Really good idea to clean out thread in both tank top and bowl before neglecting the engine for some years. I found this out same way as everyone else.
Be very careful using heat around Perspex/ Plexiglas acrylic type plastics. They melt at about 100 deg C. I made a wooden "tool" to unscrew the bowl consisting of a piece of 1 1/2" dia. wooden dowel (ex curtain rod) and turned a tapered hole in one end to match the tanks taper. If you wet the wood as well you can get a surprising amount of grip. Could also use fine wet and dry paper to grip the bowl and then polish off the tank OD with Meguiars paint cleaner or a Perspex or polycarb. polish.
To unscrew stuck alloy Mills cylinder heads I use two pieces of 1/4" MDF into each which I have carved spherical recesses which roughly match that of the head. Then very gently nip it up on a 4" bench vice. For really old untouched for half a century genuine Mills I heat the head with a small butane torch first.