MY odemeter quit working and I purchased one on Ebay , the milage was close but does anyone know if a 1999 Civic SI records this in the ECU to prove to future buyers what the true milage is ?

 

No, but when swapping odometers or clusters with different mileages, you should contact your DMV to determining the procedure for recording such changes, so that you don't end up with issues when the car is being sold. It's usually just some simple paperwork.