Broken Photodiode - how to find a replacement?
Any IR PD will work as they all have similar characteristics, except this is radial with daylight blocking dark plastic filter.

keywords for search...

PIN photodiode IR radial side view digikey (or RS or Mouser etc)

The 1st likely match I found was,

http://www.digikey.ca/product-detail/en/lite-on-inc/LTR-516AD/160-1032-ND/121725




  Looks very good - but isnt it necessary to take care what wavelength, resistor values etc? - Peter Dudikoff 



maybe , but I have some confidence it will work well from tons of research in these products in the past, the chip size is pretty standard and the RESPONSIVITY is typically 0.4A/W  or 0.4uA/uW, wide IR spectrum, low speed app.


there are others to choose that may be better or worse, as long it is stock , pick 2 types , if you like, (cheap)




http://www.digikey.ca/product-search/en/sensors-transducers/optical-sensors-photodiodes/1967048?k=&amp;pkeyword=&amp;pv16=10757&amp;FV=17840001%2C17880004%2C17880006%2C17880007%2C17880008%2C1788000a%2C1788000d%2C17880013%2C17880014%2C17880015%2C17880017%2C17880019%2C1788001f%2C17880022%2C17880023%2C17880024%2C17880026%2C1788002a%2C1788002c%2C1788002f%2C17880032%2C17880039%2C1788003d%2C17880043%2C17880048%2C17880049%2C1788004e%2Cfff4001e%2Cfff803c8%2C1f140000&amp;mnonly=0&amp;newproducts=0&amp;ColumnSort=0&amp;page=1&amp;stock=1&amp;quantity=0&amp;ptm=0&amp;fid=0&amp;pageSize=25

2nd option above  0.55 A/W 
spec  http://www.excelitas.com/Downloads/DTS_vtp7840.pdf

Check Anode pin.