i fitted a MC21 front end to my 16. its an easy swop just fit the 16 stem into the 21 yoke. the 18 front should fit the same way. the clip ons are too low so i,m looking for a set of 16 ones to get bored out to fit the new forks.
i also fitted a MC18 swinging arm, wheel, brakes, etc fits in no trouble at all. just make sure you use the R2j arm not the 5k.
if you fit a 18 or 21 front end to your 16 you will loose the speedo drive. i,m using a mini moto speedo at the moment, but i,m looking to fit a 21 engine cover and speedo drive.