1999 Polaris 600 RMK

If the 600 is a big block engine, the 700 and 800 will also fit in the chassis with few modifications. To do it right, you should get the ECU for the engine you want to run, there may be differences in the electronics.

VES stands for Variable Exhaust System. An engine with VES will have a black exhaust-activated valve on the front of the cylinder that opens with RPM to give better performance at low RPM and at high RPM. The blocks are similar in the VES vs. non-VES engines, but again, the electronics changes. There are different flywheels for some of the engines as well.

Used engines are going for 700 to as much as 1500 bucks, with the VES ones a bit higher than the non-VES. You may be able to take the block to a machine shop and have them drill and tap the bolts out for a lot less cost.

What kind of modifications would I have to do if I would put a 1999 polaris 700 engine into a 600 chasse? What is the ECU? Where is it located?

The ECU is the electronics control unit. It's the computer/brains of the system, telling the coils when to fire and at what degree of timing. It's a black rectangular box about 3x4x3/4" with a wire harness plug in attached to it. Unless the part number is the same from the 600 to the 700, there are differences in the electronic signal that can create problems.