* Mounting Points: The engine mounting points on the frame likely won't align. The 1980 XS650 likely has a different frame design to accommodate changes in the engine, like the addition of a rubber-mounted engine.
* Electrical System: The electrical systems between the two bikes will be significantly different. The 1980 model may have a more advanced ignition system and different wiring harness, making compatibility a major issue.
* Transmission: While the transmissions might be similar, there could be subtle differences in the mounting or splines that make them incompatible.
* Exhaust System: The exhaust system on the 1980 model would likely not fit the 1971 frame's exhaust ports and mounting locations.
* Other Components: There might be other incompatibilities with components like the carburetor, airbox, and even the swingarm.
Possible Solutions (with challenges):
* Custom Fabrication: You could potentially fabricate custom engine mounts and modify the frame to accommodate the 1980 engine. This is a very complex and time-consuming project that requires extensive welding and fabrication skills.
* Find a Matching Frame: The easiest solution is to find a 1980 XS650 frame. This is the most practical option, as it will be designed to accept the 1980 engine without major modifications.
Bottom Line: While it's technically possible, it's extremely difficult to swap a 1980 XS650 engine into a 1971 XS650 frame. You would need to be a highly skilled mechanic and have access to specialized equipment and fabrication tools. It's generally easier and more cost-effective to find a frame that matches the engine you want to use.
