Don't know about a HGT gearbox, but any 1.2 5 speed or 6 speed gearbox will slot straight in. Fairly sure this is ditto for the mk2b boxes as well but will need confirmation from someone else.
I've just procured myself a new (well, used) 5 speed box from a scrapyard for £100 from a 03 reg 1.2 8v with only 40k miles on the clock so it is fairly easy to get a decent box for a reasonable price. If you have the money a reconditioned box can be worth it either.
Hope this helps
Matt