i might suggest a dial indicator to set the timing,presuming the timing is adjustable.
as a CR model i would think it has electronic ignition and hopefully either the Pulsar or the stator plate can be adjusted.
here is a video on the use of a dial indicator used on my 1975 CR250M1
i used a Fluke 88 in place of a buzz box since it has breaker points and i had no buzz box available at the time.
a CDI requires no buzz box so if the timing is adjustable and it has marks to use then you are set but if you alter the timing a dial indicator is a must.here is where you can get one and you may need the extender if your engine is air cooled.
http://www.hvccycle.net/tools/
scroll down a ways to near bottom and you will see the dial indicator and adapter set for air cooled engines.
i think yours is liquid cooled and only the CR500 was air cooled that year if memory serves.
if the timing is in spec and can't be adjusted the only thing i can suggest is wearing a boot and build your leg muscles.that is all i can offer here.
