There should be two horns on a Roadster Jon, high and low note located under the tray at bottom of grill, or above if spoiler fitted. If either fail you will be left with a seemingly weak single horn. If you remove the number plate support (two bolts after removing plate) the grill will drop out also and access will be easy from below or above.
Had to change one on our Mini recently for same reason, not the easy to access one of course
I wouldn't bother trying to rebuild the OE horns. They are sold as matching pairs on eBay for peanuts. Might be an opportunity to upgrade to relay operated air horns though.
