There are two different styles of cylinder head for KLX models as far as I can tell, which I will call “old” and “new”. The “new” style heads are easily distinguished when by the presence of a bolt in the middle of the cylinder head on the clutch side of the motor, and by having a 4 bolts valve cover instead of 3 bolt.
The “old” cylinder heads were used from 1994 to 2020 on the KLX 250 models, and from 1997 to 2007 on the 300 models. On-road models have provisions for air injection, and EFI models have a slightly different intake boot and provisions for a coolant temperature sensor. The same intake cam and cam gears have been used from 1994-2020 on the 250 models, and from 1997 to 2007 on the 300s. The exhaust cam has had a few P/N updates due to changes to the automatic compression release, but the cam grind is apparently the same. All bikes sold with the old style head have 11.0:1 compression.
The “new” cylider heads have been in use on the Ninja 250SL and Z250SL models sold in europe and asia that use a KLX based motor. The SL models have different cams, with a different cam gear that may or may not be compatible with the “old style” cam gears, and do not have an automatic compression release. The SL models have 11.3 to 1 compression, up from 11.0:1 compression on the KLX 250.
The updated cams and slightly increased compression allow the 250SL models to make a claimed 27.6 hp and 16.7 lb ft of torque, up from a claimed 22.8 hp and 15 lb ft for the KLX. The power curve is shifted toward top end power.
The updated heads have been introduced elsewhere in the KLX lineup starting with the 2020 KLX 300. The 2020 and 2021 KLX 300R models have the exact same cam P/Ns as the 250SL, also do not have compresion release, and also feature slightly increased compression compared to the previous KLX 300. Anecdotal evidence seems to indicate a nice performance boost over the KLX 250 and older KLX 300R models, but no official power figures are listed on kawasaki’s website.
As of March 5, 2021 I’ve only seen one parts diagram for the 2021 on-road version of the KLX 300SM. It appears to use the “new” style head, but the cam and cam gear part numbers are different than the off road version. The cams have automatic compression release, and at least looking at the diagram, the cam gears look identical to the “old” style heads. From what I’ve read, the cam profiles are the same as the 2020 KLX 300R. However, I’m unable to confirm whether or not the cam gears are different than those found in the “old” style heads.
I’m currently unaware of the exact differences in design between the two styles of cylinder heads. All bikes share the same 61.2 mm stroke, only differing in bore, so the new heads may have a different chamber design or the bikes shipping with the “new” head may have a different piston to account for the compression differences. All bikes claim to run on 87 octane (91 RON) gasoline.
All the different versions of the KLX cam use 32 teeth gears and the same cam chain p/n, so even though the cam and gear might be different, the 250SL or 300SM cams may work for older KLX owners looking for an upgrade. A 300cc+ cylinder would be recommended unless the KLX is purely street driven, since there will be less low end torque with the 250SL cams otherwise.
Compatibility is pure speculation at this point, since I have not had my hands on a Z250SL or 300SM cam. I’ve included cam part numbers on the parts reference page for those willing to try, feel free to open an issue here and let me know the results.