Hgf and the cambelt snapping are 2 different things! If you had a huge hgf, why were you still driving? And you must have been doing motorway speeds to assume you did that much damage to the conrod and pistons!! If the belt snapped i can only say it was down to the driver not checking to see if they needed changing! The only way i can see 'just happening' would be if there was an il leak onto the belt, causing it to weaken.
Engines from th 75, Freelander and Rover 218 as well as the MGF and MGTF will all fit, if that helps to broaden your search a little.
Whereabouts are you based, and what MGF is it? Im looking for a donor car, so if you decide another engine is too much hassle, I may be able to buy yours and you can put that towards a replacement F (instead of scrapping it!)