SmartStart button part number

I don't think it would be a separately purchasable item. The pushbutton power on the Prius is highly integrated with the HV ECU and the transponder ECU and the power ECU working together to identify the fob, enable power to the car's electrical, then re-certifying the fob to enable the HV ECU to actually move the car. This 'package' would not be easy to defeat,, as it is like trying to hotwire a computer network.

SKS is not remote start. You have to be within a few feet of the receiver for it to accept the fob. SKS is an additional component that gets its authentication via a bit longer range RF, and relays that to the transponder ECU as if the transponder ECU read the fob ID directly from the slot, though it is actually a different ID.

Now there is a question as to whether the fob as a rolling code for the slot based RFID or if it is a fixed 48 bit code. Much speculation here, but nothing positively documenting this in my view.

I don't know about the Lexus, but I would hope and imagine that the engine ECM would require the same authorization from the transponder ECU that the HV ECU needs in the Prius. I really doubt there is an aftermarket installable package. Remote starts probably add to a cars vunerablility.

Yes, once the car is powered on to READY using SKS, someone could drive off with the car without having the fob with them, but would no longer be able to power it on again once it was turned off. Also, adding fobs to the car's memory requires one of the already known fobs to be used, so they wouldn't be able to add their own fob to the list if they drove off.

Now if the fob is put in the slot, it gets locked in until you power off the car, much like an ignition key gets locked in when the engine is on.

No security system is infallible however, its more of a matter of how much trouble it is to defeat, and if that trouble is worth the return.