I belive that Doreen Edgington still holds the Lister records and is supplying the factory information as for build dates, original shipping dates and customers.
You can email her  at

Ddlwf123@aol.com.  or. enginbooks@aol.com

Yes those are symptoms of a weak cylinder or cylinders and coyld be due to weak compression or weak pump(s) or failing injector. I would first crack lines one at a time, if your problem is on certain cylinder(s)  that will isolate it. I am not familiar with the TX but if it can be run with the exhaust manifold removed you can then see which cylinder(s) are smoking. Another way to tell if its oil smoke or half burned fuel is the smell. Oil smoke stinks but half burned fuel smoke not only stinks but is very irritating to the eyes.

Your emails went to spam for some reason, replied just now

Yes, those are the parts I posted. You need a tappet and the top piece that receives the push rod. It is possible to remove the tappet without major disassembly but a bit tedious. You remove the clamp holding the tappet guides then twist and pry the exhaust tappet guide upwards and remove it. Then reach in and get the tappet. Do reverse to install.

 If I am understanding you correctly the parts you need are called the exhaust valve tapppet assembly. Circled in the attached picture. Do you also need the tappet guide? or just the tappet? I have these in stock in Ohio good used and new.
I assume you are talking about John Ferguson? He moved to Central America many years ago, guessing 10?

As you probably know I run a belt driven 15KW head with a 1115 China horizontal that is running  a tick over 1500 RPM so maybe 16?? HP. My reasoning for the large head is as your post here,  It starts our motor loads effortlessly. When I decided to spend on a whole house stand by the wife said I want to be able to keep house same as grid. Our large starting loads are same as most everyone else's, well pump,  A/C,  Plans were to add a flywheel to the generator shaft but that turned out to be unnecessary.  Outside of initial cost there is little reason to not upsize the head for a stand by set. Years ago there were many posts about more drag from the fan and bearings but lets face it they were splitting hairs, same with the serpentine belt deal. Maybe had some importance with a 6/1 running 18 hours a day every day but who does that now? I don't know of anyone. In a stand by situation its pretty much immaterial. Being as I am an engine mechanic and pretty much electrically illiterate I also figured the large head working at half capacity would last longer??  The set up is now about 13 years old and outside of the always bad bridge rectifier replacement it has been trouble free. Largest surge load it has seen was last summer when we lost power, AC was on, wife was washing clothes, lights etc and I needed to run my 6.5HP air compressor. To my supprize the compressor fired right up with a puff of smoke from the 1115. At that time I didn't have a load monitor which I have since installed.

It's a model D, yes petrol.