I personally have NEVER had good luck with a 4 hole spacer.
I have about 1.5 inches of open spacer on mine.( tarantula) you have enough carb there. so IMO the best bet would be 1.6 rockers. you would most likely gain even if you used 1.6 only on the intake. and tightened up the exhaust lash a bit.
try some lash changes at the track.
Good luck to you .
cant suggest anything else that doesnt involve a lot of money (porting and bigger valves)