Why do you want rebuild kits? Are your jets destroyed? Assuming the brass jets are okay then they should be reused. Given that, the only thing you need are the O-rings and the O-rings are common parts as used on lots of other Kawasaki bikes using Keihin carbs. The sizes listed below are for the KZ750E and H bikes. You can do a cross reference using Partzilla to verify they fit on your 305. I did a quick spot check and the numbers I checked all cross reference.
Edit: the one part that's good to replace, other than O-rings, is the float needle. The needle is shared with tons of different models and Keyster makes the best aftermarket needles. OEM is best of course, but Keyster is decent. Most other aftermarket needles are poor because the spring inside is too weak to hold up the float.
Regarding the diaphragms, it's not a trivial task to change them. Are you familiar with the process? I did some recently on some Mikuni slides and it took quite a bit of doing using a lathe to get the slides properly anchored. JMB diaphgrams are quite a bit easier to install. You might want to check those out.
jbmindustries.com/Dimensions.html
Pilot Screw: 1.2 x 2.6
Pilot Plug: 1.5 x 2.8
Fuel Transfer Tubes: 2.0 x 5.5
Drain Screws: 1.4 x 4.0
Float Bowl: 2.0 x 67
