will ltd 750 engine fit in my 79 kz 650 frame?

i have a 79 kz 650 im looking for some more power out of her i have a 1980 ltd 750 motor will it fit in my frame and will i be gaining anything from doing this?

i also have a spair 650engine i plan on doing a 810 kit but dont know if i should use the 650 lower end or just use the 750 i have

The 750 will drop right into the frame. They share the same cases and swap a lot of parts between the two motors. You'll gain 100 cc, and like the old saying goes...there's no replacement for displacement!

Other than bore size, they are the same exact motor. The only difference is that it will say "738" instead of "652" on the side of the motor!

"I also have a spare 650 engine I plan on doing a 810 kit but don't know if I should use the 650 lower end or just use the 750 I have."

Using the 650 would require using a Dremal tool to clearance the cases for the larger cylinder block, then the big bore kit and the 750 head if (only certain year 750 heads work) it has the provisions for using the 650 cam chain and idlers. Using the 750 only requires the big bore kit. A lot less work and hunting down the right head.

Here is an article that explains what head is needed...