[dsraven]

check and clean all connections, like you did already. ensure you lay hands on ALL connections, positive and negative. everywhere. battery, starter, fuse block, grounds, ign switch, starter and the "stepon" starter switch, dah dee dah.
do a voltage check before and after start up. after start up should be more than before start up. you gotta rev it up to get the generator working while you do this because the generator doesn't charge a lot at idle. if you generally start it and let it idle for extended periods to "keep things lubed up" then the battery may not be getting charged well and that can be a problem right there.
if your battery was dead for a bit or low on acid it may have sulfated the plates which means there is a coating on the plates so the chemical reaction can't happen on that area. charge the battery with a fast charge to help break that sulfate off if that is the case. the adverae affect here is that stuff that breaks off the plates becomes sludge below the plates, which can move around while driving and end up shorting out the plates in that cell. have the battery load tested. ensure it is fully charged and then allowed to sit and rest for a bit and then apply a load of 1/2 the cold cranking amps for 15 seconds -the voltage would not drop past 5 volts. use a carbon pile tester if you have one available or check around at starter alternator generator shops to see who may know something other than what google says on their phone. most auto supply shops have a digital unit that is calibrated for 12 volt batteries so the school kid behind the counter will likely say your battery is toast no matter what.
next check the starter draw. you will need a few electrical tools to do this like an amp meter and how to hook it up. an amp meter with a pick up probe clamp that wraps around the battery cable to the starter would be good, then you will get actual starter draw. disconnect the coil wire high tension lead and ground it so the engine won't start. this probe clamp can also be used to check the amperage the battery is receiving when the engine is running by clamping the probe around the battery positive cable. if you have one of those carbon pile testers you can start the engine and apply a load using the carbon pile to see what the generator is putting out through the amp probe. google sun AVR meter for the old fashioned unit. usually the AVR will say what the amp load is as you crank down the carbon pile and when you see the voltage drop that will be the generator output at that rpm.
check the battery cable to the starter, do a voltage drop test to ensure the cable isn't becoming a big resistor. if the cable is getting warm then there is resistance somewhere near the warm spot or else the cable is undersized or the starter draw is more than the cable was rated for. if you have one or several of those braided ground cables you may also want to check that for a voltage drop. whatever voltage or amperage passes through the pos cable also has to pass through the neg cable or ground system. check the shielded/insulated cables for lumps along their length as those lumps are likely corrosion inside the cable which is a resistance. a damaged cable can have corrosion around a break in the insulation wrapping. crimped on termi9nals also are a great spot for corrosion inside the crimp or down the cable shielding if there is no seal on the insulation at the end of the cable.
if you boost with 12v on a 6v system you may cause damage to other things in the system that don't really like to be over voltaged.
unless you are tied to a 6v system you could upgrade to a 12v system and use the original 6v starter as long as the engine tune up is good so the starter doesn't have to be cranked for longer periods. the old 6v starters will handle 12v really well. google the swap procedure for what it takes if you are interested. 12v alternator will charge well at idle and the 12v will spin the 6v starter slightly faster than 6v did so the engine may even fire up quicker. just a thought. battery, alternator and electrical plug, all bulbs, horns, and a guage resistor set up would be required. 12v radio as well if you have one or at least a voltage drop system of some sort.