Can I replace hood covers on Shimano ST-2300 brake/gear levers?
An ST-R9150/R9170 hood is not going to work on an ST-2300 shifter.

R9150/R9170 is Dura Ace DI2 - electronic shifting.  This is an ST-R9170 shifter:



ST-2300 looks like this:



Note the significant differences, especially in the "cap" area, where the ST-R9170 hood extends forward over the top end of the shifter, where on the ST-2300 that same area is a hard Shimano logo.

Your best bet is to look for ST-2300 hoods on large online retailer and auction sites...