I completed this swap last month. I had the same questions as you and didn't find any clear answers so hopefully this helps. I did the swap on my 2021 Gen 1 Alloy Levo SL. Keep the hardware with the bike, not the motors, at least for the alloy frame there was unique hardware that stays with the bike. The skid plate fitment was the biggest issue, both plates had to be modified and took a lot of grind and fit checks to make them work. My son is barely 48" so I modified the stock SL 1.1 cranks to 130mm, fits him great now. 155mm cranks are still WAY to long for a 24" kids bike IMO.
- how to remove the limitation
* A Specialized dealer can reprogram the ECU to the standard Levo SL software, however my dealer would not do this and discouraged me from doing such for warranty purposes. If the software doesn't match the bike/motor some dealers might not be as friendly for warranty replacement if needed. I purchased a SPEEDi device. It has worked great and gives me 22 MPH speed capability. The display does not show the correct speed but I use my Garmin for tracking rides.
SPEEDi - E-Bike Speed Limit Increaser by NLS Components
- if the stock 155mm cranks are usable for adults
* As far as strength; they are the same forging as the adult cranks, pedal hole is just drilled and tapped at 155mm. They will be just as strong as "Adult" Levo SL crank arms. Depending on your height and riding terrain, crank arm length is really subjective to personal preference. I am 6"1' and ride mostly flowing XC terrain, I wanted longer crank arms. I traded my 155mm cranks for a set of 165mm arms with a member on a Levo SL Facebook group.
- compatibility of the chainring with non-Transmission AXS
* I have been using X-Sync 2 chainring(s) on multiple bikes with full Shimano XT 11 speed drivetrains. Has worked great. Obviously you cant mix & match with 12 speed Shimano, but the 11 speed parts work fine together. If you are asking about standard AXS, the chainring will work great with any SRAM 12 speed group set.
- if there might some kind of preselection going on for the motors going in kids bikes
* I don't think there is. I noticed the extra torque and reduced noise from the SL 1.2 motor from my sons bike. According to the Specialized app I get the full 320 watts of output on the kids 1.2 SL motor. I like the new motor, its an improvement but its not completely life changing.
Now that I have done the swap and figured out some of challenges I am happy with it. However, if I were to go back in time with my current knowledge, I would likely forgo the swap and leave both bikes as-is.
If you have any specific questions, let me know I will try to help you out.