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Hello, i have a Briggs 16 hp L head engine, I use it for a pulling tractor, it needs rebuilt, I was wondering if I would be able to make it into a 20 HP engine. Do they use the same block, heads, crank, pistons? do I need to bore it out? Im not finding a lot of info on these engines, but i love my little Briggs

IT all depends upon which short block your 16Hp engine is
The thing we need is the model number
The first 2 numbers are the engines capacity ( approx ) in Cubic inches
If your engine is a 40 incher then you should be able to get more out of it but if it is a 35 incher then tough luck.
The easiest way to get some extra grunt would be to go to a mower race shop and get a hotter cam .
There is a limit on over boring and a bigger limit on upping the compression.
However you can fill in the head to make actual inlet & exhaust tracts much along the lines of what HD did with the K series
You run the inlet along the bottom of the cylinder so it directs the incoming charge at a tangent which scavenges the exhaust better
That makes a sort of funnel shape to direct the exhaust out of the cylinder

As I have been told many many times, the limit to the available Hp is the thickness of your wallet.
However on a governed engine it tends to be a bit of a zero sum game as you still have the transmission to deal with and adding 4Hp while it sounds like a lot does not add that much extra torque which is your pulling power.

Bert, does boring one actually give an engine more HP?

It can
What it generally does is restore the original Hp that has slowly dropped off.
What it will do is up the torque.
Got a couple of old 500cc motorcycles that I removed the sleeve & run big pistons directly on the iron cylinder .
Don't go 1 mph faster but does have a tone more grunt so uses a lot less fuel despite one of them going from 500 cc to 720 cc
Welding & profiling the head did push up the Hp particularly as one engine went form 5.7 : 1 to 7.3:1
No use trying for CR's much greater than 7:1 on a side banger because all you do is burn the bridge region of the barrel .

Hmm, a 35 inch "L" head 16 hp B&S has me curious?
Walt Conner

What brand of 500cc engine was that, suzuki? honda? I would be willing to try this

1942 Wm20 BSA linerless CAST IRON barrels from 1948 & 1952
Nothing made in Japan post WWII will have iron barrels save the Meguro then Kawasaki W 1 & 2 .
British bike & most USA ones had cast iron cylinders with austelitic iron or steel pressed in liners .
The occasional one had a liner with the alloy fins shrunk around it .
They are easy to tell because the head is through bolted to the crankcase so it sandwiches bore .
The iron was low grade & quite porus .
Advances in metallurgy to produce aero engines led to far better processing & quality control to the point that liners became unnecesary .