To install such a kit, you must pull the top end. Acquire cylinders with flat decks. Press the sleeves out of them. Bore the aluminum to accept larger sleeves. Bore and hone these to accept the larger pistons. Enlarge the spigot hole in the case to accommodate the larger sleeves. Ride only short distances and very gingerly to avoid heat-related problems. For a 102mm piston, a sleeve OD of about 106mm would be required. A deck flange with an OD of 110mm would be called for, with a depth of at least 2mm. There's room. One would only need to remove material from the cases down about 20mm.
What would it cost? Reckon a hundred fifty dollars for cylinder cores. Another hundred fifty per sleeve. Sleeve into cylinder, a hundred fifty per. Modify cases. See alternate means on bigtwins.org. Another hundred. The piston kit: $900. Total: $1650 - tentative. Maybe more, maybe less. Modified gaskets.
As the machine work and engineering are about the same, other sizes would cost similarly.