It sounds like your prop is probably too aggressive so you can't go slowly at low RPMs.
If you put on a smaller prop you may lose speed on the upper RPMs.
They do make a trolling valve for the ZF45A for around $1,800.
However some people think that the gear might be a bit too small to use a trolling valve. But ZF Marine does sell one for it.
Sometimes it makes more sense to purchase a whole new gear with a trolling valve (like a ZF63A) as adding a trolling valve on to a gear purchase is only about $400.
J.D.Tags: ZF, ZF45A