Go get a cheap caliper measurement tool from Sears or Harbor Freight. Disassemble the entire crank & chain ring assembly. Measure the dimensions. Lengths and diameters. Get a magnifying glass and count the TPI threads per inch. Once you’re certain about what you have then, go browse eBay or Amazon or the hundreds of other on line sources for parts. I can usually find exactly what I need on eBay from Asian sources for a low price and no shipping costs. Although it usually takes a long time to arrive from Hong Kong.