not affiliated with these guys at all, i just think they have a cool product at a relativity good price,
there is a few fuel injection kits dedicated to the 250 ninja.
here is one
Small Engine EFI Kit - Small Engine EFI | Engine control
and here is another
MBE Motorsports inc. : Kawasaki Ninja 250cc PNP EFI conversion kit [EFIPNPNINJA] - $498.89USD
easy and bolt on. they also sell single piston kits for somewhere about half. would make for a killer briggs vanguard setup, super tunable as well. it would be an easy way to eek out the last little bit