Altimeter shows 100 foot drop when power is added. Is this a leak in the static system or broken altimeter and how long will the fix take?
The mechanic will do a static system leak check.  This is done by applying a sealed off suction source to the static port, then applying an inch of suction, causing the altimeter to rise a thousand feet, then holding that for one minute (I do it using a big syringe connected to the blanked off static port on my homebuilt).  The altimeter is supposed to not move more than 100 feet in 1 minute (that's the rather generous Canadian requirement - not sure of the FAA but I assume it's similar)

If there are no obvious static leaks, and the static port itself seems normal, the mech will probably swap out the altimeter and see what happens.  Or, swap out the altimeter first and then try it out.

The leak check will take about 30 minutes, and the altimeter swap-out a couple hours.  Shouldn't take more than a morning to resolve.