Welcome to OCC.....
First thing I would check is the Battery Cable connections. This would apply to both cables and both ends. Make sure they are clean, bright, tight, and down to bare metal. This is essential for getting the power to the Starter.
Next, I would have the battery charged up and have a reputable shop "load test" the battery. If not up to specs, then replace the Battery with the highest "CCA" rated battery that will fit in your battery compartment.
Now, lastly would be the Delco-Remy Starter/Generator itself. The possibility exists there could be a problem with this. If you suspect so, take this to a reputable Repair Shop and have it professionally examined. If there are issues with it, by all means, spend the money to repair it as opposed to buying the cheap Chinese junk you see on the internet.
Do the above and report back.