How do I route this belt . Old one broke and fell off so not sure how it should be fitted

Mtd has a pretty good website for their parts. Go to MTD Products Landing Page (http://www.mtdproducts.com) and put in your model number. You want to look for the illustrated parts diagram link on the right. If you click on the deck listing you should be able to figure it out.

If you have trouble post the model number here and I'll try to get the picture for you.

Here is my stock answer for most any tractor. https://www.askmehelpdesk.com/tools-power-equipment/mtd-riding-lawnmowere-belt-diagram-137312.html

Hi,
MTD offers no easy way to link you directly. Look under the seat and write down both of those long number and letter combinations(model and serial) and go to:

PDF Manual Web Archive (http://manuals.mtdproducts.com/mtd/public/list.jsp)

And input that info. You get your manuals in return!
Peace,
crigby