Hey bud, really depends on what you want to spend, and what you can still find...
1st option is to strengthen the frame you already have by bracing it, im sure someone can link a thread on doing that..
Otherwise there are plenty of options just finding a nice one can be hard, in the day the top picks would have been , Five0, Red baron, Sano, Billetware and the like but they are getting harder and harder to find in good condition as most aren't made anymore, I believe fast50's still do a good 50 frame, my personal fav is the billetware cradle mount which my 88 is built up on now but I got mine from the states, another option is the original thump pro cnc frame its pretty trick looking and can take a beating, bare in mind all of the above mentioned frames are going to be a bit heavier than stock..
gpx are a good fork mate and will handle what you want, they basically a copy of the zokes, just realise that 660mm will mean you will need to jack the rear up a bit with an extended swinga and spring to get the bike to sit nice, the xjr frame would need to be strengthened id say if you were planning on going big on it, I got my frame from the states for about 600$ second hand
By the sounds of it mate your looking to build a full mod bike, id save your pennies and not go with the xjr, instead try and find a mod bike built and buy a complete bike instead of buying something that your just gonna upgrade everything on anyway