How does the engine sound?  Does it seem to be running correctly, just not developing full power?  Or, is there a misfire, or perhaps running too rich.  You mention one cylinder appears wet, have you checked the plugs after a hard full speed run?  If one plug still appears wet, perhaps the fuel pump is leaking fuel into whichever cylinder it is plumbed into.

What pitch prop are you using?  Someone may have changed the prop and installed one with too much pitch which is just bogging the engine down at high RPM.  The only other thing I can think of is that crazy “economizer linkage”, the horizontal thin rod that runs from the throttle roller back to the vertical throttle shaft.  This feature allows full advance timing before full throttle opening.  The adjustment is easy to mess up, and is much easier to set up with two people.  Unless the control throttle cable is connected, then you can advance the throttle fully (in gear) and it should stay put so you can adjust the economizer linkage.

Check the easy stuff first.  Is the throttle opening the carb throttle butterfly all the way?  Is your choke opening all the way? Is the throttle spark advance properly sync’d to the carb (part of the economizer linkage Fleetwin mentioned)?  Try  swapping the two spark plugs around and see if it’s still the bottom one that’s wet.  I’ve seen bad plugs new out of the box in recent years with Champions.  Have you put a tach on the motor to see what RPM it is revving up to?  If you don’t have one, buy a Tiny-Tach or equivalent cheaper one online – link below – these hook up super easy with just a wire wrapped around a spark plug wire (no direct connection) for the RPM signal.  Note that is could be running on one cylinder – these motors run pretty smooth on one.   Try taking one spark plug wire off at a time and see if it makes a difference – you may find a dead cylinder this way (take them off when not running and then start it again or you may get zapped).   Prop pitch has been mentioned – did your friend have it on a similar type boat? or a bigger one where he may have run a low-pitch prop?  You asked about running it in a barrel.  To check out max throttle in a barrel, you need a “test wheel” instead of the prop to put a load on it in the barrel, and keep it from cavitating.  If this easy stuff doesn’t fix it, you could have a bad reed.  Have you checked compression to know you don’t have a stuck ring?   Be sure you aren’t running a short shaft motor on a long shaft transom, or vice-versa.  Here is a link to buying a tachometer:

https://www.amazon.com/s?k=tiny+tach+tachometer&crid=2UNEI46YV5T9G&sprefix=tini+tach%2Caps%2C252&ref=nb_sb_ss_sc_2_9

TJ Brandon wrote:

For prop pitch, I cannot see it stamped anywhere on the prop.

If you take the prop off, the diameter and pitch on factory props is cast into the aluminum on the prop on one end around where the hub is pressed in.

Dave