Well, it seems like you already have marked many of the potential issues with your 337D! That's a win man!.
To my understand, machine's hydraulic system or hydraulic control unit might usually the source of the error code 06-13. It may be worthwhile to check the hydraulic fluid level and pressure after replacing the speed sensor and starting motor. You should also examine the hydraulic hoses and connections for damage or leaks. If everything appears to be in order there, you might want to think about having a qualified expert examine the device more closely in order to identify and fix the problem.
I hope this helps!
