hello. I have a dead Thinkpad MB. in looking up the serial, I get a very general result: X1 Yoga 2nd Gen (Type 20JD, 20JE, 20JF, 20JG) Laptop (ThinkPad) - Type 20JD. does this mean the same MB will fit all these models, or do I have to tear the unit down to find the serial on the MB to ensure fitment?
Generally, a single laptop chassis will accept many motherboards. It is how you get the various model selections for the most part.
You would just look for a motherboard for the X1 Yoga 2nd Gen, or specific model number of your laptop (which should be somewhere in your purchase/receipt), or yes, somewhere on the motherboard. Since the system is dead, take the motherboard out so you can compare it to the pictures of the listings.
For what you are doing, just find the CPU/GPU etc you want, and go for it.