Hi new to this site.

1944 Whizzer Model H on a Lincoln-badged Schwinn - Motorcycle Classics

Perhaps no other American-made machine gave as many young folks their first taste of powered two-wheeling as did the Whizzer.
www.motorcycleclassics.com

"138cc 4-stroke side valve engine "

I did wonder that too til I did some searching.

Wiki Cross Section View, See Diagram that has valves still on top, but intake exhaust only slightly lower down from the head:

en.wikipedia.org

Flathead engine - Wikipedia

en.wikipedia.org en.wikipedia.org

https://en.wikipedia.org/wiki/Flathead_engine

Showing later model to sell one with side valve a 1949 job:

Classic Motorcycles & Scooters for Sale in Online Auctions - Catawiki

Buy and sell Classic Motorcycles & Scooters at Catawiki. Discover Classic Motorcycles & Scooters auctions filled with special objects, selected by our experts.
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MT
 
Sounds like a fuel issue, start by cleaning the tank then, change the lines to the carb, including the filter, and the shutoff valve flowing right.
I just went through this on a lawn mower, I blew the motor ( Tecumseh ) and put a Briggs in with a new carb. Cleaned the tank and would only run a bit and stop, so change the lines and filter and it started right up.................Curt
 
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