Lots of choices,, what to do? hehe
I'd get a spring and see what that does. Well we know what that does, stays geared lower, longer at any given load. The weights in the engine clutch determine at what rpm the engine operates at as the cvt gears up, and also lets it gear down sooner on deceleration. That was my issue, new engine wanted to rev but the weights limited rpm to near stock. I lightened the weights and that lets the engine operate at higher rpm. The drawback of the cvt is engine working speed is set, whether you are driving easy or hard, so you have to pick an rpm and live with it. That's more info than you need right now but I threw it in there. Guess my point is even though you can exceed stock govenor speed, it's still operating at the same rpm except at top end after the cvt is through shifting? I guess you have bands to slip unlike me..
The engine. Weakest link is valve spring retainers, thin stamped steel that can drop a valve. Then weak valve springs take second place. If you don't want to get into the block, your ideas are about right. The thin gaskets I'm guessing will raise compression from 8.5 to 9 ish. About .030" differnce in gaskets. You can also put 16 hp heads on it(smaller chambers). Some basic port blending helps the stock heads. Briggs makes a thin gasket from years gone by(before they lowered compression). George has better quality thin gaskets. With decent exhaust, these mods should help but I have no idea how much.
When I was planning my engine, I was hesitant to change the cam, telling George I couldn't afford to lose low end torque. A couple people had Al Hodge engines and the cam was too wild to move a bigfoot through the stock cvt range. But I gave in and went with Precision cams. They make all kinds of cams. NOW, if you put a cam and valve springs/retainers, pushrods, in, the engine will want to REV. That requires better connecting rods. AND they have come up with a neat rod solution that is really slick. The stock 18 briggs piston is dished, and also below deck height almost an 1/8", assume to get the compression lower. They make stock length rods and also 1/8" longer rods. Use a longer rod, shave the dish off the stock piston, and viola, you have a flat top piston at deck height. Nice compression boost, without buying pistons, boring, milling heads, or thin gaskets.(which can misalign intake manifold when done to extremes). You need some good gas but since your user name is fulleraviation I think you have that handled
Anyway my engine has more low end than stock. The cam didn't hurt a thing down low.. the compression lets you feel every power stroke. And, at 3 or 4 grand, it is putting out some power. 4500 is pretty horsey. George posted once, "anyone can bolt in goodies and have a 32-35 hp engine, no head work". He has a dyno so I assumne he's right. That's an 18/20 hp block, long rods, shaved pistions,
a 2 bbl, small chamber heads milled 30 or 40, thin gaskets. I think he likes compression, hehe. Good exhaust a must! Rejet the stock carb. The one bbl will make plenty of power.
Ok, I'm through. I'm not a fan of big heavy tires, causes drivetrain wear, can you change sprockets somewhere if you do tires? Maybe do your conservative engine mods and a clutch spring, report back. If that's not enough then you can do the full monty.
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)