150sx to 125 conversion

I don’t believe there’s a 125 kit specifically, but I believe you can buy the cylinder with the piston kit from the OEM 125 diagram. Not sure if it comes with powervalve assembly installed though. Other than you should just need the CDI. 
 

I never found anything on the cheaper side to spread the 150 power out like the 125. I just decided to dedicate my 125 strictly for moto and am rebuilding the 150 for woods racing. The pre 2016 150 motors that were bored and stroked were magical IMO. 

maybe find someone with a 125 set up that wants a 150 to trade parts with? 

This is the way.   The 105 and 150 kits are really a bargain.  You realize this when you see what is costs to piece together every comparable 125 part.  It's quite a bit more than a 150 kit.

Sorry if these suggestions are too obvious, but here is my .02

I wonder if increasing case volume with the Moose reed block torque spacer would help. The head dome can be reshaped for spreading out the power perhaps. Needles can alter a powerbands character as well. I'd say try a more linear 125SX CDI, but I recall there being blown engines from the early CDI's. Maybe those were 125SX CDI's?

Send it to Lynks racing for more top end.  Keep it a 150

Yes, the 125 cdi unit on your bike will have less advanced map timing in the mid RPM range. It should soften the mid hit. Not going to add top end I expect. It will feel like broader smooth power.

The $35 Moose reed TorqueSpacer will lower the compression in the cases, this should free up some revs. It increases case volume. 

Changing the shape of the combustion chamber($100) or using the 150XC head can spread the power out. No reliability issues to be had. A very standard tuning item.

Nearly free to raise the cylinder with a thicker base gasket. It'll move the power upward a bit. But really the porting specs are needing to be altered. The tuner doesn't have to max top end.

Myself, I've gone with not touching the engine, but rather installing the new STIC Super X carb. Unlike the metering rod type Lectron and SmartCarb, the STIC is adding significant power to the entire RPM range and the throttle response is next level. I actually installed the slowest turn throttle tamer cam G2 makes, for the track! Your clutching gets greatly reduced, BTW. I mix it up now with faster riders on big 4 strokes and my 200SX stock makes less peak HP than your bike. Pulls new 250F's. This is nothing like an exhaust system or a collection of bolt-on mods. It's more like a 25cc big-bore ported to perfection and with a much quicker powerband. STIC SuperTorque is the company. Been using their products for about 4 years. No Choice! YAT

Pretty sure Moose does not make the Torque Spacer for the KTM 125/150 engine.

Depending on the route you go, if you don’t end up needing the 150 CDI let me know. I might be in need of one I’ll have to double check what I have here.