Pete have you ever had your Simplex engine "4 stroke" at any point in the operating rpm range, idle to say 5,000 rpm? I actually try to induce this wee bit of 4 stroking in my 2 cycle motors for cooling. Running a bit rich at a mid range rpm level that is easy to reach by just a blip of throttle or regress back to by letting off throttle from easy cruising speed. As an arbitrary example let's say 4 stroking begins at 2,500 rpm and continues until it smooths into 2 stroking at 3,500 rpm. The result is a good pull from idle to 2,500 and then a narrow band of 4 stroking is available for cooling, which can be quickly transitioned through to a smooth 2 cycle cruising speed. In my youth I ran Bultacos & quickly learned 2 cycle tricks, which aren't tricks at all but necessities to make antique 2 stroke tech work in desert heat. Keep cool fuel moving in and hot air exiting quickly and most importantly, continually. Keep the revs up, no coasting with a dead throttle, keep revs coming by gearing down on slow stretches. Quite different than operating a 4 stroke, no? Of course the plug doesn't foul as readily either. I actually like my 2 stroke plugs to read a bit rich. Playing the heat range game with plugs along with carb adjustment and timing is also a good way to reduce heat.
Have you changed compression by truing the head or cylinder deck or both, if so a thicker gasket can be employed to lower compression and heat.
Use of too thin a base gasket can also alter compression.
Finally timing. Advance is good at lower rpm's on a 2 or 4 stroke, but the 2 strokes actually run better and cooler with retard at higher rpms. Many street & racing CDI units are designed with this in mind. Perform better and live longer. Motors like the Simplex shouldn't require much advance to perform well.
Premium (no corn fuel ) won't hurt the motor & is quick to check out. Usually if the motor isn't detonating, pinging the gas is fine. I break in with semi synthetic Lucas 2 cycle oil at 40:1 then switch to 20:1 when they start coming to life. Older engines might require a higher ratio. I believe the new ash free synthetic 2 stroke oils are a key factor in my engines running so long and are more than woth their cost.
Case inducted motors are normally considered to cool better than piston port engines and are more efficient. Since the piston skirt is seizing I'll mention 2 things that I've not covered. Trimming the piston skirt and/or thoughtful drilling of the piston body with small holes to allow the cooling fuel mix to lubricate and cool the cylinder wall continually through the interior of the piston. Drilling is quite common as is trimming of the two I prefer drilling.
Racing motors of the dry sump 4 stroke variety, which run on & are lubricated by fuel oil mix and some operate continually at 12,000 plus rpm.
Exhaust heat buildup can also add significantly to piston,cylinder and head temps. You wouldn't believe the number of cat converters and mufflers that are plugged up though "almost new" ....might run pretty good but be generating internal heat.
I also can't help but believe that heat is at the root of your motors problem with seizing. Without miking the rotating parts and checking the bore and clearances, all of which create friction and heat, I'd be at a loss at this point for another cause.
I'd suggest acquiring a good laser temp and record both head and cylinder temps at various locations.
Just a few observations and generalizations on 2 strokes in general, not Simplex specific. Have fun, Rick C.
