OK, so the battery is 12V, you have a new one, engine cranks, but… any noise at all indicating firing? Any puffing out of the exhaust?
Keep in mind-- it's not good to run the starter for more than 15 seconds or so- they will get hot, and may suffer internal damage if they're run too long. Starters are only designed for short-duration running.
Perhaps first thing you could do, if you have no indication of firing, is to pull a plug wire, and, holding it with gloves, hold the spark plug wire about 2 cm from the end of the plug (or some other grounded metal part) and have someone crank for a few seconds. What you'd like to see is a good spark jumping from the plug wire to the metal of the engine. You might also be able to hear it "snapping." I often use a #2 phillips screwdriver inserted into the end of the plug wire for this, if the wire has a deep rubber cap on the end.
Or, you could pull a plug out of the engine, re-attach the plug wire to it, and touch the base of the plug to the metal block of the engine -- but NOT near the spark plug hole, as there may be fuel in there that could ignite-- and again look for indication of a spark inside the spark plug gap at the bottom.
If you get no spark, the problem is most likely in the ignition circuit; if you do see spark, start checking to make sure you have fuel getting to the cylinders.