are we talking a 12 volt system?
its doubtfull your 5.5 hp engine will make 150amps
100amps maybe...
look for a 110-555jho prestolite- leece/neville alternator
it is a large frame alternator, with front and rear ball brgs for long life
a regulator that is piggy back mounted and doubles as the brush cover.
http://www.prestolite.com/literature/alts/PP1131_110-555.pdf
from the graph it will need to turn 2200rpm to make 100amps, but figure on
2500 to account to a hot rating. very important to pay attention to direction of the fan rotation.
so run your engine at its full 3600rpm gear down to ~2500 about a 2:3 ratio
your best served by either dual A belts of good quality or a serpentine belt
forget aluminum pulleys and cheap single belts it aint gonna happen.
the alternator shaft size is 7/8" which is common off the shelf stuff if you
go with dual A belts, check surpluscenter.com for pulleys.
btw, these alternators are common on hd trucks, available on monthly sales at hd truck dealers such as navistar, peterbilt, kw, freightliner etc. for about a buck and amp, so figure about 160bucks or so, which is cheap for a new hd alternator without a core charge.
if after you get it, it doesn't work out for you as you would like, let me know
these alternators are capable of some interesting things.
also if you are handy you can pick these units up for next to nothing maybe 5 bucks for scrap, when they fail as they sell new so cheap no one will rebuild them. no money in it. usually it is just the brushes that wear short, and they are servicable from outside the case a 5 minute job at most.
bob g