I tried every permutation that I could think of to find a JD 100 Lawn Tractor. Same with the 34 in Mower Deck. Maybe they don't have the older Lawn Tractors in the online parts catalog. Either way I couldn't find anything. Sorry.. I did try.
Maybe someone more adept on the JD site will be able to help......
In the John Deere Parts Look up Rudi posted, above. Type in 100 where it askes for model. Then scroll down to item # 1472. It lists the belts by John Deere part number - not size and length.
If the old belts are not present - it's to the John Deere dealership.
I went to the Deere parts site and searched for 34. Got back a listing for "1008 - 34 Rotary Mower - 15Apr03". Expanded out, it showed "7 - DECK DRIVE ASSEMBLY". That showed a deck belt of M41642 which you should be able to cross reference several places.
I presume that is the correct mower, but you probably want to look at the diagrams yourself to see if it looks right. It is a 2 spindle deck with 17-1/4" blades.
Ok, so how did you guys get the results. I oriiginally tried 100 and got nada, squat, zilch, nuddink That is why I asked it if was a 100 or an L-100. Same thing with the mower... figured out it was a 34 inch by the model .... typed in Mower Deck, 34 inch Mower Deck, 34 inch, 34, Mower .. always came back with nada, squat, zilch, nuddink ... ad nauseaum...