Cateye Strada Cadence will do it for you. $35. Or you can keep your gizmo but install it the right way. Cateye will require same installation techniques.
First of, to keep magnet on spoke you must wrap it with tape so that it's on there securely. Vinyl, 2.5 mil, automotive, works great. Check out my post in bicycle mechanics.
Cadence magnet could be secured to small cog using tie wraps.
Then, all wires must be taped, again using vinyl. It keeps them that much more secure. Then you can use the tie wraps where it's recommended. Using vinyl underneath the tie wraps also protects the paint. Also use vinyl underneath the mounts, etc..
If you going for new computer go cateye strada cadence. To operate you click on the bottom of the body, and it cycles through several displays. Always showing your current speed, whether accelerating or declerating. Shows dist1, dist2, cadence, odometer, clock, elapsed time. Attached to a bracket that you mount on stem. Can be secure to the bracket using vinyl tape or something similar, so it always stays in place. Useful when you like to carry your bike in doors by grabbing the stem.
To fix your paint, use automotive was and polish. If it's scratched you could tape it with vinyl.
Don't ignore the cadence. It's like your most important reading.