To fit in the PR250/300 engine into the engine bay, you have to do the following:
1. You have to round out the end of the down tube that extends beyond the twin spar part of the frame or the front of the engine will not be able to fit due to the counterbalancer. I cut a "V" in there with a cutoff wheel with an angle grinder and then rounded it out with a Dremel tool using a cobalt grinding bit. You can see the area that needs to be removed in the photos below with the circled areas.