Here is a link to a place that their specialty is small engine replacement. they also have lots of parts for small engines, such a pulleys, bearings, belts etc that is not readily seen on-line for some reason)
You will have to nose around on-line to find contact info. If you give them the EXACT engine ID numbers you want to replace, they make all the decisions. If it's a Briggs L-head which is NLA. they will tell you replacement options and even give them the equipment model, for example and mower and they can tell you of any mods that might need to be done, such as exhaust, choke, throttle, etc., USUALLY. If the don't just ask them about mods needed.
They have a toll free phone number for tech support somewhere.
They were email only sometime back, but finally got a tech phone number.
They usually have to review and call you back.
http://tulsaenginewarehouse.com/Located in Oklahoma and Florida.
The town is Oklahoma is called Kansas which is misleading and the name Tulsa is also misleading, they are 100 miles east of Tulsa and been in business quite awhile.
Even if you do not buy from them
you can get their recommended model replacement info and go searching elsewhere knowing what you need.