Using a captured tank you like doesn't only grant you its strengths, but also its weaknesses. Re-equipping the vehicle with domestic components is a partial solution, but in some cases, eliminating the weakest link is just not possible.
"To the Chief of GBTU USA, Major General of the Tank Engineering Service, comrade N.N. Alymov
October 5th, 1943
If a tank's engine is removed at the regimental repair yard, no matter how good the rest of the tank is, it cannot be restored and leaving it in inventory would be dishonest. As practice shows, it is impossible to find vehicles in which main components are damaged but engines are not, as the engine is the weakest and most easily worn out part of the vehicle.
I ask you to confirm your agreement to write off vehicles from repair base inventory when the engine is removed.
Factory #37 director, Zelikson
Factory #37 chief engineer, Kozyrev"
The letter doesn't straight up say what tank this is, but the date and factory make it obvious. In the summer of 1943, factory #37 was just ramping up production of the SU-76I, an SPG made using captured StuG and PzIII chassis. Seems that German engine problems weren't exclusive to their larger vehicles.