78CJ5 ... sorry for the answers in form of additional questions, but what you want has mayn solutions and you'll get the best answers from such a wide variety of possible solutions by further specifying your question.
If you want to use the SF KL4 head on 1x123 and have a 2-stage switch (for high and low mode) you are pretty much stuck with the McGizmo Aleph system. Not that this is bad as those are high quality components.
The key to success is the Aleph tailcap housing the McE2S 2-stage module making your light 2-stage ... by adding a resistor to drop the converter in the KL4 out of regulation and reduce brightness in the process. The so achieved low-mode is unregulated. Those are the possible Aleph tailcaps available:
To choose the brightness of your low mode you'll need to specify the value of the resistor in the tailcap ... the higher the number, the lower the low mode. please know that there is a difference when using CR123 and Li-Ion here ... Li-Ion will be brighter with the same resistor number.
Please have a look at the McGizmo forum here and read the Aleph-FAQ (linked in the sticky threads) to learn more about the resistor values.
The Aleph tailcap isn't a clicky though ... it's a twisty. Rotate for low mode and twist further for high. In addition to that you have the momentary button at the rear end to achieve either low or high with a press of your finger. Basically it works just like the SF L1/L2/A2 switch.
Then there's the 1x123 body you need ... and there the variety is big. As the Aleph System is modular and will is completely interchageable with SF E-Series lights you are free to choose here. You can use a SF E1e body or the Aleph 1x123 body as well as some other custom options.
Here's a Aleph Standard Tailcap on SF E1e with a McLux PR head.
And those are the available Aleph bodies in various sizes. Color is HA-NAT (as pictured) or HA-dark slate (almost black).
If you don't insist on the 2-stage and want very small instead you can go for the VitalGear bodies ... teh 1x123 in your case:
If you are further interested in the modular E-Series components there's a thread in the McGizmo forum called "Aleph compatible parts" which is referenced in the sticky section ... which contains all available E-Series compatible parts for your personal flashaholic's LEGO ...
Any further questions? Just ask.
bernie