I fear you'll find that the sun-damaged plastic is embrittled through and through. So, having polished it to a fare thee well, you'll have bought yourself some time. The moulded plexiglass fore hatch on our Morgan 27 broke in two one day when slammed shut by wind. It had survived similar treatment for more than 30 years. It was somewhat crazed but really did not look so bad. I took it into a Philly plastic fabricating shop (to have it glued) and the fellow behind the counter diagnosed is a as sun-damaged by listening to it as he rapped on it.
Ever since, I've been unwilling to step on plastic hatch covers that show crazing.