Go with a decent 2nd hand 1998-2003 FZS600 engine. Anything with under 30000 miles should still have plenty of life left in it. There's at least 8 engines on ebay at the moment between £100 and £400, tho price mainly reflects condition & mileage.
Obviously I cant be certain but item number 160712267043 (2003 and 17k miles) seems like a good option IF no exhaust studs are broken off
The later (FZ-6) engine runs via fuel injection instead of carbs. Wiring is almost certainly different, so I'd avoid the massive headache of trying to fit it. Besides, its heavily based on the early R6 engine which has more power at the top-end, at the expense of midrange, so it won't be as good for normal road use.