I'm sure Barry will offer you a great approach but, if it helps, I did do a similar retrofit on my AV36 - after it had already had a motor retrofitted!
Rear of ribs added to spar. Changes TE so ribs not just butt glued to spruce LE. Full length 1/64ply top and bottom encapsulates the lot and is very strong.
Centre section almost done and sitting at 450g.. TE encapsulated in strips of 1/64ply quite rigid compared to the original design. Centre section sheeted as well for piece of mind as there isn't a lot of meat on the bone round that area. Will do underside as well in the middle.
Brass joiners and premade strut fixings on ply plates. They can be installed after sheeting. Need to now build the wing tips.
Centre section sheeted on top and aluminium fittings glued in place. Remarkably accurate fitting which isn't my hallmark. I'm happy with a 3mm error but everything is spot on.
Struts next but not before I do the sheeting under the wing. Outer sheeting wil be done once the outer panels are completed so any joining issues are easy to address