Royal B-25
The kit required you to make the decision on retracts before you started sheeting the nacells and the bottom of the fuselage. It could be converted but I think it would be a ton of work. You have to modify two of the nacelle formers before you installl the retract. The Spring Air gear I used was the firewall mount type and drilling the formers to get the gear mounted would be the biggest part. You would end up removing a lot of sheeting just for access.
I think I'd fly the plane the way it's built and if you really like it put it up for sale again and build another with the retracts installed in the building process. The Royal kits are now marketed as Marutaka kits and Hobby Barn in Arizona sells them exclusively. Hope this helps you make your decision.
Jack Devine