Kropatchek wrote on 12/31/69 at 16:00:11:Short answer>>>>>>>>>> NO
Long answer:
The 5 speed gearbox has 2 axles for the shiftforks in the transmission. The 4 speed has one. Hence the crankcase is different.
You can however put the '98 engine in the '87 frame.
(Or use the bottom-end of the 98 and the top-end of the 87)
BUT
The ignition parts have been changed over the years.
Differences: the rotor, pick-up and the CDI
The easy way to install the '98 engine in the '87 frame; install the '98 CDI together with the engine.
Greetz
Kropatchek
What he said.... I put a 98 engine into a 87 frame. I fought a "backfire" problem for a week until I found out the CDI (ignitor, computer, whatever you want to call it) needed changed too. After that ran all fine.
So, swap the motor and the ignitor box (had to solder the connector off the 98 wire set onto the 87 wire set too) and you got your 5 gears.