$660 if its for a runner! Thats absolute theft!
I got a cherry 87 zilla about a year ago, like others have said, do reliability upgrades first, then build power.
1.  Check compression. Should be around the 150 range at sea level.
2. If there is Any reason to tear down the top end, use a wiseco piston, 99 rm 250 wrist pin bearing, And billet thrust washers.
3. Pull the stator side cover and check the flywheel for any play.  It should be tight.  If its not, you need to split the cases and have inserts, bearings, seals, maybe crank trued and balanced if its bad.
4. Best fix for the head is getting it o ringed, and cut for proper squish, coolant passages etc. Iceracer, HPR and maybe some others can do it for you.
5. The stock 87 zilla carb has some issues. Contact HPR for advice/ mods.
6. Only use race gas.  The ethanol and inconsistancy of pump gas is bad bad bad for any 2 stroke.  These are awesome bikes but not cheap to run if you want them to stay together.
Im sure there are other things to improve, but these are the basics to address before you start to add real power.