No,No!! A very few of the last run of 88/99 motors were reputed to have been fitted into 650 cases as 88/99 cases had run out. 650 crank and rods are a different stroke and big end size to the 88/99 motors. 88/99 cases are identical to each other and are pretty much the same as 650 cases except 650 cases have a bulge at the back to give clearance for the extra crank throw and big end bolts. 650 barrels and 99 barrels are interchangeable with a bit of work to give pushrod clearance, they are not identical though . 650 ones may be stronger. 88 motors have been bored and fitted with 99 pistons to make a 525 motor. My 99 runs with 650 pistons as that was all i could find , a bit confusing . I have considered cutting and welding a bit more space in my 99 cases and fitting a 650 crank and rods which will also allow me to use my 99 head barrels and the 650 pistons. Now I have really confused you. There is not much wrong with the 88 motor ,tough as old boots ,smooth and will rev and be up with bigger bikes at the end of a run.