Kit?
Stick with the Payen gaskets, figure on buying all the bulk rings, lay shaft, lay shaft bearings, the anti rattle parts and a few other bearings that "once" it's apart you will see.
I'm unaware of any kit as the gearboxes are rebuilt mostly on specific needs.
Also pick up a new input shaft bushing to install into the back of the crankshaft. Pick up a Clymer’s Sprite & MG Midget shop manual if you can. The Clymer’s has been out of print for some time but Ebay and used motorbook dealers will have copies from time to time. Great for step by step with pictures.