On my Skidoo/Rotax 2 Strokes I was told by LA Sleeve you need a Minimum 2mm / 0.07873992" Cylinder Wall. Find a Site that makes Sleeves, they give the ID/OD of the Sleeve. Most Sled Cylinders can be Over Bored 2mm+. Like my Rotax 277 (72mm) Bore, Sleeve ID is 72mm/2.835" and Sleeve OD 3.77". So 3.770 - 2.835 = 0.935"/2 = 0.4675" Wall = 11.8745mm. For Max Bore if Sleeve OD 3.77" / 95.758" - 4mm = 91.758mm if your Head/Cylinder Bolt Patern will allow it. With the 277's (4) Bolt 100mm Bolt Pattern 82mm is the largest Bore you can use. I have seen a 670 Big Bore 82.5mm which use's a (6) Bolt 100mm Bolt Pattern.Can you bore gx390 to 92mm without resleeving it?
Do you know what the Max these GX390's can be Big Bored too? 96mm is the largest I have seen.Honda offers 3 oversize pistons .25mm .5mm .75mm they are typical rebuild parts.