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 a larger sleeves. Bore and hone these to accept the larger pistons. Enlarge the spigot hole to accommodate the larger sleeves. Ride only short distances and very gingerly to avoid heat-related problems. For a 106mm piston, a sleeve OD of about 110mm would be required. A deck flange with an OD of 114mm 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.More to come.