The 1988 520-H is model 31-20OE01 and uses an Onan P220G-1/10540A (20=hp, O=Onan engine, E=Eaton transmission, 01=parts list identifier)
The 1989 520-H is model 31-20OE02 and uses an Onan P220G-1/10540B
Serial number 15683 or 15883 fits better to other serial numbers in 1988 but could be 1989.
You can download manuals fromhttps://lookup3.toro.com/request/request.cfm
Enter the 31-20OE01 model number and type none in the serial # box.
For a list of the attachments and their long model numbers go tohttps://lookup3.toro.com/partdex/index.cfm?xCaller=Toro
Enter the 31-20OE01 model number and that will take you to a page where you can get them. Also try the first link for manuals using the long model numbers.
For illustrated parts lists go to http://www.partstree.com/parts/
Put Toro in the 2nd box and 31-20OE01 in the 3rd box.