Ok, there are a few parts that are interchangeable between the mightyboy (550cc) and the hatch (800cc)... I wouldn't be able to tell you off the top of my head but there is a couple.
I just bought an Exedy F8B clutch kit for about $150 so i think i did ok there.
The difference between the 800cc and 550cc gearboxes are the drive ratios... the 800cc box has a slightly taller final drive ratio so you can cruise along at a pretty high speed for the car with minimal RPM.
As far as i know the 550 and 800 boxes are interchangeable but i'd be sticking to the 800 gearbox.
The only swift struts that can be made to fit are the SA model swift struts, i'm not sure how much it would cost to get them modified to fit the hatch but i think it would be cheaper and easier to get the old ones rebuilt.
You could get a swaybar custom fitted to the back of the hatch, i know of a bloke that's got one made up for on his mightyboy and he says it handles much better than standard.
If you're not already a member of The Australian Mightyboy Owners Network - Home
, you should really have a look, there's heaps of info on there and plenty of guys that know more than me about mightyboys, hatchs and altos..
Hope this helps,