1:35 scale Italeri kit - circa 1980, basically stock out of the box. I did “screen” the rear air intake with sheet brass screen I had laying around and framed it with evergreen brand plastic strips. The mud is celluclay tinted with waterbased craft paint. I purchased this kit along with a Panzerbefehlswagen I (Italeri kit - late 70’s early 80’s too) on ebay for $18.00 for the pair a while back.
When you said OOB, I was looking for that ugly piece of rubber that serves as an exhaust cover. I like the replacement brass part you made. I also like the mud and the track sag [^]
I have always used celluclay for my groundwork so yes, it does work well for mud on a vehicle. Always remember to mix in your colour (waterbased only!) and add a teaspoon or so of either Elmer’s white glue or some kind of waterbased carpenters wood glue when mixing up the Celluclay. Adding the glue will insure proper binding for whatever vegetation you add and will insure that “stick” needed to adhere it to a tank hull.
I crazy glued the tracks to the tops of the return rollers and about halfway back between the last return roller and the rear idler wheel I drilled a small hole and inserted a pice of small, thin paper clip to hold it down… old Shep Paine trick.