I think you are really off track here! ;0)
You would want the gearbox on as those things turn about 2700 RPM and a higher speed would give more power with less stress on the engine.
Remember, you are reversing the power flow so the gearing speeds the motor up not slows it down. That's a good thing in this case.
I have played with these and you are not going to get consumption amperage out of them as a generator. It all depends what their rating is to start with. Could be 150W, could be 350. As a generator their are losses because you are not doing what they were designed to.
If you are looking at a 6 HP engine to drive these, you are looking at the wrong sort of unit. Use a regular car alternator. A 6 HP engine will laugh at that which means you can gear the thing up with a 6-8 pulley on the motor which means you can run the motor nice and slow and quiet. Even at fast idle you will produce a heap more amps with an alternator than you ever will with one of these motors. You will have problems coupling them to a 6HP in either belting which will put side thrust where it was not intended or in getting a coupling to do what is probably an 8mm on the motor and a 19-25 on the engine.
The other thing with the motors being brushed is they will wear out infinitely quicker than an alternator. Where I am, those motors are also worth much more so you'd be better off selling the motor and buying an alt by far. It will have a pulley and of course is designed to be belt driven. Put a pulley on the engine, make a bracket or base to mount the motor and alt which you can put your battery on and switches, lights or wheels and you are there.
Also a single wire alternator has the charging regulation built in. You'd need a solar controller or something for the motor.
If you were to overload the motor, which is highly likely with an inverter, you'll fry it real easy. With a modern twin fan enclosed alternator, you are going to have to try REALLY hard to accomplish that and I'd still say you'll more than likley fail.
You could easy get 70A out of an alternator, not going to get that with a motor. If you want more 120+ alts are common now. If you were after all out power you could put a 4" pulley on the engine and drive 2 alts off a 6HP motor. My lister 6/1, My China diesel and the 5 HP petrol Kawasaki I have will all drive 2x80A alts to their limit. You'd cruise at 120A which will give you a max of 1200W+ on the inverter and get your batteries charged up fast as possible.
You should look at your inverters and their rating to work out what they need. There are a lot of variables such as the inverters efficiency, the actual voltage you put into them ( they will usualy go 11 to 16V) and other things but a good safe rule of thumb on a 12V system is conveniently a factor of 10.
That means if you divide your inverters output in watts by 10, you'll get the amps you need to feed into it to run it to full output.
If you have a 1000W inverter, you'll need 100A to maintain battery level. If the inverter is 600W, you'll need 60A and so it goes. Most likely you'll be able to get a little more but you'll never run your battery down or come up short if you calculate on this.
Even if you got 20A out of the DC motor, that's only going to support a 200W load which isn't a lot.
And before any of the electrical purists start nitpicking, no, the 10X factor is not absolute BUT, it is an easy to work factor and rule of thumb that has a measure of built in tolerance to all the usual variances in the equation.
My suggestion is to look at this another way and use a regular car alternator rather than your motor. It's basicly the wrong tool for the job.
It will work but with severe limitations in output, longevity, control, setup and real cost.