Yes, pretty much every one of these bikes needs their wiring gone through and cleaned up. That includes wiring AND all the connections. If doing all that doesn't solve your voltage drop issue and you still want to do the regulator relay mod, here's how to wire it up .....
Pulling the power from as close to the battery as possible is best so that you avoid as many connections (and possible voltage drops) as you can. Do take a voltage reading at the positive battery terminal and then again at the solenoid connection. You only have a short (maybe 6") battery cable there but it could be bad, or maybe it's connection points are.
As far as relays go, there are several types. Here's a link that will educate you about them .....
https://www.12voltplanet.co.uk/relay-guide.html
Any of the common types you'll need are readily available at most auto parts stores for a couple bucks.