Often times the issues become identifying the model, or identifying the part. 
I hope this helps you to find what you are looking for.

Here are sites for your Gravely:

• If you have the Gravely serial number, you can 
  identify the the model and/or year: 
  Serial Number Breakdown

• Misc. Gravely stuff
  New England Gravely Club

• Find a replacement Kohler engine for a Gravely:
  Kohler Replacement Engine Guide

• Pics of old Gravely equipment
  http://www.homestead.com/rototiller4/Gravely.html

Antique Tractors in General...

• Antique Tractors
  http://www.antiquetractors.com/

• Hemmings Motor News
  http://www.hmn.com/

• Midwest Tool Collectors
  http://www.mwtca.org/

• Antique Tractors Online
  http://www.antiquetractorsonline.com/

• Engine Ads
  http://www.enginads.com/

• Vintage tractors in general, good reference:
  http://www.simplicityva.com/page-4.html

• Antique Small Engine Collectors Club
  http://www.asecc.com/classifieds/