The 800 motor will physically fit, as it is simply a bored out 750 and thus the hieght is the same. However, you will need the computer control box from the year bike you get the motor from, which will set you back another $100-400 depending on if you want a new one or are willing to chance a junkyard one. Rremember that the trans and motor are one unit, so you are really taking a chance if you buy a used motor: the thing may run fine but shift badly, or the clutch may be bad.
Where are you located? A guy on another website offered be a good 750 motor a couple months ago, and he may still have it in his garage. The bike ran great and had low miles, but he wrecked the front end.