Maybe I missed something, but did you say it was running OK at WOT and does it idle fine?
Mikkis basically have three ranges of tuning, most people never have to change slid cut-aways and almost never needles except for extremely fine tuning for elevations or a particular track condition.
The LS jet regulates idle to about 1/8. It should have some adjustment if jetted correctly with the air screw about 3/4 to 1 1/2 turns out (if it's the type that monitors fuel and not air) This is sometimes one of the differences between so-called 2 & 4 stroke carbs, aside from an accelerator pump.
From about 1/4 to 3/4 it' the needle clip position (and possibly the taper). Basically all of the needles have the same largest diameter for a given model and it's all about the taper diameter/length, or even multi-slope length. Note: the cut-away of the slide can also effect the 1/8 to 1/4 if it's really off, which doesn't really happen too much.
From 3/4 to WOT it all about the MJ, nothing more.
Check out my Spec Sheet when I was running a 70cc w/4-reed 20mm VM. I think I had about a 160MJ and 20LS, std needle & slide in the #4 position(?) That should get you in the ballpark. If not, it may be starving.
What I would recommend is to start by pulling the fuel line off, and if the flow is in question, pull any fuel filter(s) out and ensure that you're getting a full flow to the carb. Many of the older bikes have restricted fuel flow from sediment/aging/swollen petcock seals, or weren't designed to flow enough fuel for a larger carb. This happened on my Extreme Urban. The petcock sealing disk had nearly closed up. I had to drill the rivets out and thoroughly clean it, reshape the sealing/directional disk and tap it for some 4-40 screws to re-assemble it. After that, it was flowing fine and helped to resolve some early jetting problems.