While there is no theoretical minimum diameter of the magnet hub, it should be a hub with a diameter that is near to the diameter of the front bearing snout. That's because the sensor mounts to that snout, and the sensor needs to be quite close, (.020 to .040" more or less), to the magnet. A good magnet diameter is around 1/8" or 3/16" and you can get magnets from lots of sources online. K & J Magnetics is one.

You want to use a piece of music wire or hard steel to pin the piston, and the pin diameter should be the same as the width of the piston ring land. You drill a hole in the piston several thousandths of an inch smaller than the pin, and then put a thin coat of JB Weld on the pin and carefully drive it into the piston. You must be very careful to locate the pin in the piston so that when the piston is in the cylinder, the pin is located in an area that does not have any ports that the piston ring crosses. (Transfer and exhaust ports) You then VERY carefully grind the pin down so that it is halfway down in the piston ring land. This is where the notches in the ring go, over the top of the pin, which is only half the depth of the ring land. If you don't have the pin ground down enough, it will hold the ring out a bit, and you will break the ring when you go to assemble the engine, and/or it will be pushing the ring out against the cylinder wall, which you do not want. It's too bad Frank is too busy to do this piston mod operation for you as it is a bit tricky. I definitely recommend getting a new, notched ring from Frank Bowman. Your original ring is undoubtedly worn, and Frank's rings are the best. His will come already notched for you, and most engines respond with a power increase with Frank's rings. Not to mention easier starting and better overall running. The ring gap spec on this engine is .003", which is about the width of a piece of paper. Your ring gap is undoubtedly much larger than that, and that costs power. As they say, "Never try to hop up a worn engine!"

When I hop these engines up, I do a special mod to the intake manifold, and enlarge the ports. When I do so, the pulse hole for the pulse that operates the fuel pump in the carburetor gets very close to the intake tract, or in fact gets broken into. I fill the pulse port in the manifold adapter with JB Weld, and then provide for an exterior pulse to operate the fuel pump in the carb. That is the line on the engine in the picture that you asked about. As far as the velocity stack, yes, that is the way I recommend installing it; with the larger opening to the rear. People often think it should go forward to get some ram air effect, but the truth is ram air only gives about 1% more power at 100 mph, and usually causes problems with accurate carburetion. You don't want ram air.

Well, long post but I hope it answers all your questions. Here is a link to a post I did several years ago about hopping up a Homelite 25cc: http://www.rcuniverse.com/forum/m_26...tm.htm#2621860

Good luck and have fun,
AV8TOR