Honda cb100 motor sezied can i rebore and fit larger piston and ifso what. Thanks

In adition to the above coment, which i must say is spot on, the posability of a used barel and piston if a new piston is not available could be worth considering, other things you may wish to check on that particular engine is camshaft seizure, in a uk bike it runs directly in the machined surface of the cylinder head, oil failure due to blocked oil filter has severe consequenses, located on the l/h side of the engine under a 22 or 24mm spaner head cap just below the gear change lever, this is a washable filter retained by a cap and spring, correct replacment is inperitive. are you located in UK? HOPE THIS HELPS YOU

That cylinder can be rebored. You would go to 1st oversize, 2nd oversize, 3rd oversize, etc., as needed. You won't know what size will be needed until the cylinder is bored enough to clean up the scoring. The problem will be finding a new piston and rings for the bike. You can try babbit's but no promises on a bike that old. You may be able to take the old piston to a Honda shop and have them match it up to a newer bike. You may get lucky.
www.babbittsonline.com/pages/parts/viewbybrand/default.aspx

I seized my mx360 up 3 times....Each time...I used a 3 blade hone on a drill to bore it out 1mm at a time until I got rid of the score marks...Then, I used a inside micrometer to measure the top, center, and lower bore diameter's...and bought a piston to match my bore and taper measurements. If you only seized the top end, your crank and cam should be fine. Use new oil, and a new filter and you'll be fine..Also, you might want to re jet the carb power jet itself to make up for your new (larger) bore....from 100cc to 110 or 120cc etc. There is a formula you can access to figure it out when done boring...Also, there is a standard guide you can look up to tell you your NEW engine's cubic centimeter displacement...Good luck...and remember the 500 mile break in on those new rings and piston....