Some thoughts
No doubt someone who knows better than me will be along in a bit...
I had a Mk2 Hawk back in the day , & I think the Hurricane pistol sight is the same. They are still available. Don't know if it'll fit a Mk1.
According to Chambers, the spares no for the rear sight leaf is OS 337 but they are out of stock.
Plastic soldering is a bit of a black art, especially on a small part. I've done it on m/bike bits, using a soldering 'gun' with a specialist tip, & a plastic knitting needle to melt into the break. But I don't think it would work, unless you are experienced or have a very steady hand.
Disassembling the rear sight? Take lots of pictures from different angles, & remove one screw at a time. & take pictures at every stage. My digital camera is one of the favourite items in my toolkit now, whether working on push/motorbikes or guns. Oh, & watch for small springs making a break for freedom - don't ask me how I know!
HTH
Bru
Webley Mk3 x2, Falcon & Junior rifles, HW35x2, AirSporter x2, Gold Star, Meteors x2, Diana 25. SMK B19, Webley Senior, Premier, Hurricane x 2, Tempest, Dan Wesson 8", Crosman 3576, Legends PO8.