Hi Egor,
Mushroom lifters are a must if you have a high lift camshaft [lift above .150"]. I have test camshafts with lift above .212" but must be ground a special way to clear the flywheel throws. If the base of the lifter is wider [mushroom lifters]the lifter stays on top of the lobe longer and simply means the valve is held at maximum lift for a longer period of time. And I am sure there is some difference in duration, however it must be small, because I haven't been able to detect much difference with my equipment. However the motor will run much better with the wider lifters. I have tested 4 different types of mushroom lifters, one set special made for me by a friend using a stock WC-1 set, and machining part of a B & S lifter, then pressing them together, but it takes a world class machinist to pull that off. The second version was a stock set of vintage 1/2" lifters that I cut down to fit into the lifter bores [.393"] on the new edition motors. Third set was from Woodstock Whizzer Works, and the last was Whizzer's edition. Some of Ralph's were slightly oversized, but a little time with some #220 & #600 sandpaper easily corrected that issue. Whizzer's version can be modified to make a good set, so I suggest you purchase them through you Whizzer dealer, and I will gladly share the information needed.
The slotted arm presents a problem when trying to correct the loose connection at the motor. If at all possible I would suggest you obtain the arm with the single hole if you want to improve your clutch system. I conducted several tests today concerning the excess side play in the clutch arm & pulley. If you want a "cheap" test fix to see how much improvement is possible, simply remove the bolt, knurl the bolt to .471" on the part that passes through the motor and .475" on the part that passes through the hole in the arm. However the follwing must be true, first the bolt must pass through the arm, then through the motor, and the nut must be on the dip stick side of the motor. Secondly the arm must not have the slotted hole. Locking the clutch in place can cause 3 problems, first it is hard on the clutch bearings as the belt tightens and loosens, and secondly in my tests it caused rear spokes to break [only spokes attached to the sheeve broke]. The third problem is not common, but I have seen it happen twice, the corner of the motor snapped off where the arm was attached. In both cases the automatic clutch bearing ramp locked on the bearing, froze the clutch onto the mounting bolt, and locked the rear wheel.
An easy fix for leaks at the cover compression cam is to install an "O" ring. I usually grind a small area around the cam where it contacts the cover on the inside, then install the "O" ring in the groove.

Hope this information helps,
Quenton