Since you didn't provide your location, I could not design with parts that you have easy access to. The parts shown are readily available in the US.
The circuit I am providing is good up to 7A; if the alternator on your machine puts more than that, we will have to modify.
The nominal regulation voltage is 7.1V, which is right for a lead-acid battery. With the bike parked, and with the battery voltage less than 6.5V, the circuit draws less than 500uA, so can be left connected while the bike is parked for a month or less. If you are parking the bike for more than a month, I would remove the battery from the bike, or otherwise provide a means from isolating the battery if the bike is inactive for months at a time.
When the bike is running at high speed, the battery fully charged, no lights turned on, then the regulator must dissipate almost the full output of the alternator. I dont know what your alternator can put out. You should measure it to make sure that it is 7A or less.
The bulk of the wasted power goes into heating R8, the 1Ω 50W tubular ceramic wire-wound resistor. Look at the plots. The red trace is power dissipated (Watts) in R8 as a function of the excess current being shunted (the X-axis). Since at 7A R8 dissipates 50W, it must be in good thermal contact with the steel frame of the machine.
Likewise, the green trace is the power dissipation in the TO220 NFET M1. It peaks at 12W at the 3.5A point in the trace, so the TO220 package must be bolted to the machine frame using a heatsink washer mounting kit.
The blue trace is shows the regulation voltage as a function of the current being shunted.
Give some feedback about how this circuit works out on the bike. Maybe we should include it in the "completed projects" section of this web site. Be prepared to take some pictures of the installation.