Juan - just as a point of reference, go to the forums at iboats.com and
look at their FAQ's. There's a good one on troubleshooting an engine
that won't start.

Since your starter works, it could be compression, could be fuel, could
be timing, could be spark (any others? I'm an amateur).

Much about procedure on everything I mention below in this newsgroup
and the Iboats.com forums.

Compression testing is very easy and should be done regardless on any
motor as a baseline, even if it's running well. You'll not only get a
clue if your engine has good compression, but in the future, you'll
know if it's changing for the worse (or better). You need a $25 tool,
or borrow one. Best to use same tester for future tests because each
one will tend to read different, and you want to know if it's a real
change or just a tester change.

For spark, first and foremost get a new set of CORRECT new plugs gapped
properly (don't assume previous owner had CORRECT plugs in it), the
basic test is to take out the plug, ground it against the engine and
see if it's sparking. A better test of the entire system is to get an
actual adjustable spark tester and test the power of the spark at each
plug. Make sure it's adjustable. This is a $10 tool available almost
everywhere, or you can make one. You can adjust the gap on these
testers and if the spark's good you pretty much know the power being
delivered to the plug is good. Then you "only" need to troubleshoot
the fuel and the timing.

If the spark is bad, then you need to troubleshoot backwards through
the wires, coils, stator, etc. I've troubleshooted (?) coils and wires
before - that's really simple. Haven't had the pleasure of stator yet.

For an old engine with unknown history, I'd probably would just go
ahead and thoroughly clean rebuild the carb(s). It really is simple for
an amateur if you have a decent (oem or aftermarket) shop manual, a
correct rebuild/gasket kit and reasonable care and patience. The kits
are cheap. There's much about this on those Iboats.com forums as well.
The downside to dirty carbs is so bad (catastrophic failure due to fuel
starvation and resulting lack of lubrication) that it's just good to
start with clean carbs.

Then, finally, with a decent shop manual, it is also within the scope
of an amateur to do a "linc and sync" and basic setting of the timing
system.

I'm no pro mechanic but have muddled through a lot of this stuff - it's
just a matter of following a rational troubleshooting procedure and not
going at it willy nilly. That's why I urge you to look at that faq I
was telling you about. And as you go along each step, post for advice
on specific things.