1. Buy a meter, which will be cheaper than replacing parts without knowing what's bad 2. Put the neg meter probe, starting first test under load on the battery negative [with + probe on the battery +], and then test at the starter ground to ensure that you have a good ground for the starter...
Another option that I've done many times, get some line of the larger diameter to route to where it terminates onto the smaller barb fitting. Use a short piece of the smaller diameter tubing [a piece of the old bad one] on the barb fitting and slide the larger dia tubing over the smaller...