Cool roller!
36/10=3.6 (3.6:1 ratio) 30/10=3 (3.0:1 ratio) 3.6x3=10.8 (10.8:1 overall ratio)
30/10=3 (3.0:1 ratio) 30/10=3 (3.0:1 ratio) 3x3=9 (9.0:1 overall ratio)
The larger diameter wheel/tire doesn't change the actual ratio but it does change the effective ratio (search "effective gear ratio"). All else being equal, the larger diameter tire would need a numerically higher ratio (commonly, and confusingly, referred to as lower gearing) than what a smaller diameter tire would need. The greater the difference between the two numbers, the higher the ratio; the lowest possible ratio being 1:1 (zero difference between the two numbers). Using the numbers above, 10.8:1 is a higher ratio than 9.0:1 (but commonly referred to as lower gearing). With the swingarm suspension, you pretty much need to run a jackshaft to avoid chain slack problems. The jackshaft needs to be as close as possible to the swingarm pivot, ideally they would be one and the same for constant (non-changing) chain slack throughout suspension travel. Try to locate the jackshaft to where you can draw a straight line from the jackshaft, through the swingarm pivot, to the rear axle at mid suspension travel for minimum chain slack change during suspension travel. I'm no expert on minibike gearing but I think I would shoot for around 7.5:1 to 8.5:1 overall with the large diameter wheels and only 3.5 HP, just for a starting point (of course, your intended use should be considered: street, trail, or...).
More math examples
29/10=2.9 (2.9:1 ratio) 30/10=3 (3.0:1 ratio) 2.9x3=8.7 (8.7:1 overall ratio)
28/10=2.8 (2.8:1 ratio) 30/10=3 (3.0:1 ratio) 2.8x3=8.4 (8.4:1 overall ratio)
27/10=2.7 (2.7:1 ratio) 30/10=3 (3.0:1 ratio) 2.7x3=8.1 (8.1:1 overall ratio)
26/10=2.6 (2.6:1 ratio) 30/10=3 (3.0:1 ratio) 2.6x3=7.8 (7.8:1 overall ratio)
25/10=2.5 (2.5:1 ratio) 30/10=3 (3.0:1 ratio) 2.5x3= 7.5 (7.5:1 overall ratio)
20/10=2 (2.0:1 ratio) 30/10=3 (3.0:1 ratio) 2x3=6 (6.0:1 overall ratio)
Obviously, all of these are using your 10 tooth clutch and 10 tooth jackshaft to 30 tooth wheel sprocket tooth count. If you decide to use something different (that rear sprocket doesn't look too healthy), just substitute the numbers and use the same simple math.
Having said all of that, I would suggest a more powerful engine. A Predator 212 to keep cost down, or a 125cc Lifan pit bike engine with 4-speed manual transmission, or Lifan 200cc with 5-speed manual transmission. The 125 is long (horizontal cylinder) and the 200 is tall (vertical cylinder) so either one may require frame modification and both would require mount fabrication. The transmission on either should put the sprocket fairly close to the swingarm pivot. The Predator might fit in the frame without modification, a jackshaft or torque converter should be used to get the sprocket close to the swingarm pivot.
Edit: Just checked out your other video, looks like you have plenty of engines on hand. It also made me think of a frame mounted jackshaft which would give you more options as to where to locate the engine left/right since the sprockets can be located anywhere on the shaft, not just adjacent to each other like on the engine mounted jackshaft. You could even put one sprocket outboard of the carrier bearing (chain from engine) and one inboard (chain to wheel) if you get a long enough jackshaft (I'd buy a longish one and cut it to length needed after mockup) to fit one of those big engines you have; locate the other bearing on the other side of the frame (as typical) to minimize stress on the drive side bearing.
