NField Gear (me) sells a universal replacement that is a better unit than stock. It requires a bit of wire splicing but.....
The solonoid can be a real weak point on these older bikes. If you get much voltage drop at all they will usually buzz and nothing will happen. A GREAT battery is required. I also like to run a ground wire directly from the battery to the unit. I think that part of it is that there are other devices on the same circuit that can rob volts from the system.