There is a way of making your own. Find the internal wire between the alternator and the regulator , cut it and solder some lengths of insulated wire onto the cut ends. lead these outside the alternator to a convenient place.
Get three reasonably robust diodes (from an old alternator maybe) wire them in series between the two wires that you led out of the alternator.
Each diode has a voltage drop of 0.6 of a volt, so the three in series equal 1.8 volts.
This voltage drop tricks the regulator into thinking the battery
is 1.8volts lower than it really is, and it puts the charge in at a high rate.
You want to regulate the charge, so wire three switches, one across each diode.
With all the switches turned on, the system workes as normal, but as you open each switch you introduce an added voltage drop and the charge increases.
You will need heatsinks for the diodes.
I made one of these systems with a Bosch alternator some years ago and it worked well.
Make sure your batteries can accept the extra charge. I disconnected it when we got gel batteries.