L2 is marked R91 because it's the value of it. R typically means a Decimal on components like that. So the R91 means it's 0.91uH. Another example is L3 and L4 on the board there, the are marked 4R7, for 4.7uH. You'll see that on some Resistors here and there as well, where 2R2 means it's value is 2.2 Ohms.
D23 is some kind of Schottky diode. You could toss just about anything in there and it would work though as it's just for keeping anything from getting back onto the USB 5v rail.
Check the values of all the Resistors around there while you're at it, to make sure nothing has gone way out of spec. Some of those are precision Resistors and having them be off even a few Ohms in value would effect how that circuit Regulates the 5v down to 3v for U2 and U3 to use.
