TW200s came with 2 different wheel sprockets. 50T takes 122 links. 45T takes 120 links. If you have anally maintained your stock chain, it and the stock sprockets will be about 25% worn out. You will loose 25% of the life potential of a new chain by wrapping it on 25% worn out sprockets. If you have not maintained the chain at all, it is probably 75% worn out, and wrapping a new chain around such sprockets will reduce the life of the chain 75%. If I was in your shoes, I'd go ahead and keep the stock parts and save for a whole new set.