You will most of the parts a common between these two engines. One huge difference: the ignitions of these two engines. The 87 has the older dual power pack system. The newer one has the single power pack. When the single pack came out, there were features added, such as SLOW and Quickstart. Both desirable features. Looks like the 87 has the large 35 amp (vented flywheel) charging system. No sure if the 89 has that upgraded system or the old 10 am charging system. The 89 parts list does not show the 35 amp system, but it was marketed as a sales feature of the GT/XP models.