The Wilga should be trimmed while towing the glider.
Leave downthrust alone at first. It takes away from pulling ability. You are not after 3D neutral flight, and the engine must pull hard at low speeds.
If I were to design it, I would start out at 0° (neutral down), and about 2° of right thrust to account for prop wash.
If in the tow you must trim the plane down, add downthrust as needed.