Gentle Readers,
How do I answer all these question? Well, maybe over thirty-five years of doing it face-to-face, over the phone or online might do that. I would be called "old school" since many of the brands I learned to work on had illutrated parts lists as the learning aid.
For engines I go to the sources:
Operator Manuals, Illustrated Parts Lists & Wiring Diagrams - Briggs & Stratton

Honda Engines

Kawasaki Engines & Power Products - Home Page

Kohler Engines: Owners & Service Manuals:
Manuals & Maintenance

Not all but answers most
And for equipment manufacturers I often go to:
Operator's Manuals for John Deere Equipment

Honda Power Equipment - Operator's and Owner's Manuals

PDF Manual Web Archive
Poulan Pro - Poulan - Weed Eater - McCulloch - User Manuals

Toro - Lawn Mowers, Zero Turn Mowers, Tractors, Snow Blowers

For reference purposes I often go to (order of preference):
Outdoor Power Equipment Information

Power equipment parts and accessories from Outdoor Distributors

K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts

K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts for older Murray

Lawn Mower Parts Cross-Referencing

Lawn Mower Parts and Lawn Tractor Parts Snapper Cub Cadet MTD and More

Parts & Accessories | Shop & Find Lawn & Garden, Appliance Parts at Sears PartsDirect | SearsPartsDirect.com

Stens - The Parts Company

Wise Equipment & Rental Inc | Ariens OEM Parts | Briggs & Stratton OEM Parts | Briggs & Stratton Power Products (formerly Generac) OEM Parts | Briggs & Stratton Yard Power (formerly Murray) OEM Parts | Echo OEM Parts | Electrolux / AYP OEM Parts | Gr

Manufacturers
For access to the ARI network of parts lookup:

http://www.milacalawn.com/parts.asp

Russo Power Equipment - Home

For belt cross references:
Main Belts Reference Listing

Snowmobile Parts, ATV Parts, Go Kart Parts, Mini Bike Part, Lawn Mower Parts, Chainsaw Part and Small Engine Parts

For some Deere questions:
https://groundscare.custhelp.com/app/
Enjoy! More later, and Peace,
Clarke

Clough

Thank you for posting that, crigby!

It's extremely helpful and a boon for this site!

Thanks!

crigby

Kind Readers,
I have stored many of the pictures of belts I use on Tripod at:
OPE by Crigby
Some sections are works in progress, others are virtually complete.
Peace,
Clarke

crigby

Gentle Readers,
Some omissions are present since I did not discuss manuals:

Free lawn and garden product owners manuals, user guides & how-to instructions | ManualsOnline is the Omnibus site and,

ManageMyHome.com - Home Improvement Projects, Owner's

Manuals & Home Management Tools will cover Sears and a few others
But for repair or alternate engine sites

Honda Engines

K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts

Kohler Manuals

K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts
and I know of a Tecumseh, cannot find it. Kohler, Kawasaki and Tecumseh allow old manuals to be published. Briggs does not!

But engine suppliers have info, too. Some manuals, line drawings, specs, etc.

Small Engines - small engine - Welcome to Small Engine Distributors, The Dealer's First Call Source for Lawn & Garden Supplies. Contact us at smengdis@swbell.net or info@smallenginedistributors.net - Our wholesale website is http://www.sedkc.com & ht

Small Engine Suppliers - Your online source for Replacement Small Engines, Tools & Parts

Small Engine Warehouse - Repower Outdoor Power Equipment

Tulsa Engine Warehouse www.tulsaenginewarehouse.com tewarehouse.com webstore new site for www.smallenginebusiness.com
After all how can they sell it if they do not show it fits!

And I did leave out JDParts:
John Deere - Parts Catalog
Peace,
Clarke