Most likely the valve cover gasket. Engine specs for parts are located on the valve cover if this is a Briggs engine. A Kohler will have a white tag on the shroud.

catleys, please post some other info and numbers to go with your otherwise good description.
We need to know the specific tractor Model Number and Serial Number in order to view an online manual from MTD.
This number is usually found under the seat when seat is tilted up. Some tractors will have that info on a hood decal.
Also if you can find numbers on the engine itself, those should also be posted along with the tractor model & serial.
Once we know which chassis and which engine you are dealing with we may be able to advise you on your needed course of action.

Kohler Courage engine, twin-cam, 535CC, serial # 3811517483.

If a valve cover gasket, is it an affordable fix?

Thanks for taking the time to respond and offer up some thoughts!! Seriously appreciated?

Very affordable fix......even if you have to take it to a pro shop.
Of course we lean heavily toward DIY repairs here, but we understand (barely) how some folks prefer to "farm it out".
If you are reasonably handy and have a few basic mechanic's tools it can be DIY. If you think you want to try it you need to get to the Kohler web site and download a free copy of the service manual for your engine.

mownie...I have downloaded the relevant pages. The visual looks to be right where I seem to have a leak, so while I'm admittedly at the lunatic-fringe of being mechanically inept...I actually think I'm going to give this a shot. I've been inspired to attempt a DIY.

Is this as easy as removing the valve cover, replacing the gasket, and re-torqing the valve cover screws in the prescribed sequence and to the prescribed specs? Any gotchas?

BTW - LOVED the 'we understand (barely)' comment!!!!!

Yes. They also have redesigned the cover on these to lessen the re-occurance of another leak.

Well, some of us here are people who always seem to have some dirt under their fingernails, so we have to remember that not everybody is like us.
If you have the correct service manual from Kohler, there are only 2 pages you need to see for changing the valve cover gasket.
Page 8.7 will help in removing the valve cover.
Page 10.13 will help in putting the cover and new gasket back on the cylinder head.

Troy built Bronco - Mower has plenty of pulling power but just crawls along , no speed . Replaced both belts ..

"Troy built Bronco - Mower has plenty of pulling power but just crawls along , no speed . Replaced both belts .." A new thread?? You really need to supply more info IE model numbers, part numbers. Did you use OEM belts?

Model Number - 13WX78KS011 YEAR 2011
Upper belt - 954-0428// Drive belt - 954-0467A
Belts ordered through Troy-Bilt .
Was running slow ,belts were worn so I replaced them but it didn't help ...

Back pulley is also free to move up and down as it should be ..

(954-04208A) You have started out on the right track. A few things to double check when you have a slow ground speed: 1) the brake will not fully release. You can generally wiggle the disc back and forth. If not, take it on a "roadtest" and give it a cautious touch as it will be very hot if the brake is dragging. 2) The drive pulley below the engine is worn, cracked or glazed 3) the bearings in the VS pulley or idler pulley are binding and creating a load greater than what the belt can handle 4) The link adjustment is incorrect . 5) the upper belt was not placed behind the tab on frame (most have this tab, I can't say that all do) 6) the rear tension spring has been hooked in an incorrect location. Let us know how you do.