What parts from the iPad 6 are interchangeable with the iPad 5 parts?

The iPad 5 and 6 are not simple refreshes.

There is a new CPU.

There is a new display which needs to support the apple pencil.

There is are new chipsets supporting the Apple SIM card vs a standard SIM card.

From these 3 parts, the likelyhood of internals from a 5 fitting in a 6 or vice versa are slim to none.

The logicboard on the 6 would be redesigned from the ground up, and would probably be using the new Intel baseband and other chipsets due to Apple's current status with Qualcomm.

The chances are connectors change locations, shapes, etc, even for the basic things.