Windspirit, there is no glue that will do all the jobs you want to do.
Virtually any professional upholsterer can do that conversion for you. Unless you plan to set up a professional upholstery shop, you're way ahead to consult a professional.
Conchos require that you remove the leather covering of the saddle before setting them: in the case of HD seat pans, that means using a staple puller. In the case of Corbin and Custom Chrome saddles, that means drilling out the rivets that hold on the leather.
Few motorcycles use actual leather in the seats anymore: vinyl is waterproof, wears much better, and does not require continual maintenance. The saddles my guys make include gel packs, which form to the fanny of the rider. In the case of custom scooters, it is necessary to fit the seat to the frame: Borguette and Big Dog motorcycles can be a finicky fit.
