Could be the old engine had a
fuel pump failure, and the owners chose to add a generic electric
pump instead of replacing the OEM one. They might have just bypassed a failed mechanical diaphragm
pump or something else. Then simply ignored or overlooked the 'extra'
pump when the repower was done. Certainly if they contracted that out--the guys who put the new one in could have said "What's that? Dunno, leave it alone' and left it there.
Or, someone might have installed it as a cheap antitheft measure, i.e. if someone drunk & looking to joyride tried to start the original engine--it wouldn't run without
fuel, and the "pay no attention to me" secret switch.
If you are sure the switch makes the
pump run, and the
pump is a flow-through type (doesn't block the flow when it is off) then leaving it be should be harmless. Or you can disconnect it and run a new seamless
fuel line if you prefer "less stuff less surprises".
Can you still contact the PO to ask them what it was all about?