Would I have to modify anything to put a 87+ motor on a 1986 frame?

I just bought a 86 all stock. Ive seen 87+ motors for 300-400$. Thats not much $ per performance increase. Seems like the best place to start.

 

yes, the tube on which the front motor mount is bolted to forks to go around the pipe in the later years,(your bike has the exhaust exit to the left of this frame tube) so the motor mounts up slightly different in the front and the pipe mounts up slightly different.

 

Do you know if the jug and head from my 86 motor would fit a 87+ motor?

It doesn't look hard to modify the mount, but I would rather not spend more $$ on a different pipe.

I know the 87+ has a power valve. Although I don't know what that is, it is probably on the jug/head. I could live without that though. I'm more concerned about the transmission.

 

none of the parts swap between the earlier & later engines really. the powervalve on LT's changes the volume of the exhaust chamber to create more power on top & bottom.... the cylinders and bottom ends are completely different designs