Very carefull inspection of the head face should give you an idea if it's gone soft etc.
You can get a full kit, including head gaskets, steel shim, head bolts, and uprated ladder rail, but it's £110+ http://www.ebay.co.uk/itm/MG-K-SERIES-M ... 2eb4d2bd31
On the basis that they're pretty easy to work on, it's probably worth just trying the cheap option first, and if that doesn't work, then give it the fully monty treatment.
Sounds like a missfire, have you tried putting the old arm back in ?
webmaster said:I bought an Intermotor MAF sensor for the 75 and that appeared to be useless too. Not convinced about their quality anymore, used to be reasonable enough stuff.