I'm think that should be possible, but I know that the starter is a motor, so I may need some regulating circut?

 

I was also thinking this might help others with the ac mower generator problem, but for my purposes I just want the starter converted into a generator.

 

You need first to find the type of motor it is, on a ride on mower it may have had a battery which would be DC and this you can create a DC generator, if AC type then it is either Universal, or more likely a induction motor, the first you could make a generator with some work, the latter, not practically possible.
Max.

 

I'm afraid I don't know what type the mower was, I do think though, that if I give it a 6V ac current it should start turning, should I try?

 

I can't see a 6vac moving it, but you can try, worse case it will overload your supply, but first check to see if it has brushes, if it has it is DC.
Another way is to spin it and measure the output voltage, to see if it generates a DC voltage.
Max.

 

The starter on Tecumseh snow blowers are 120vac and are Universal motors, so you may have the same on your mower.
Max.