Tap the blanking plug home with a little Loctite 603 to hold it in place.

If your casing has a hole next to the layshaft bush, an M3x19mm screw appears to fit perfectly. Use some Loctite 603 to hold it in place.

Fit the metal spacer (larger diameter inwards) into the oil seal (metal side outwards) and tap the seal into the casing. When fully home, it should sit ~4mm in from the outer surface i.e. do not leave it flush, otherwise it will bind on the sprocket.

At this point, it is a good idea to test fit the sleeve gear and outer sprocket to check that everything turns smoothly.