What would be the effect of skinning over the gap in fixed leading-edge slats?
To answer your question, I suggest to look at the effect of slats. NACA technical report 407 studied a fixed slat similar to what is used on the Zenith CH-750, and the result of the optimum geometry is plotted below. Dashed lines are for the airfoil with a fixed slat and solid lines for the airfoil with the slot between slat and wing filled:



Lift and drag coefficients for a wing with and without a fixed slat over angle of attack, from NACA TR 407.

Three conclusions are obvious:


Drag is higher with a slat for no benefit in lift (even a small reduction, actually) up to an angle of attack of 15°. Don't let the scale of the plot mislead you: Zero-lift drag for the slotted wing was 0.0229 compared to 0.015 for the plain wing, an increase of 52.5%.
The maximum angle of attack has been increased by 9° by the slotted wing, from 15° to 24°. This translates into an increase in the maximum lift coefficient of 34.6% (from 1.297 to 1.751)
The stall behavior has changed from benign to outright nasty by the addition of the slot.


While the airfoil studied in NACA TR 407 is quite close, it is not exactly what happens once the slat is removed: While the outer contour of the airfoil did not change in NACA TR 407, those builders removed part of the wing when removing the slat. Therefore, wing area would be reduced and the loss in lift will actually be even greater. NACA TR 400 hints at a lift loss of more than 40%, which would increase the minimum speed of the Zenith CH-750 by about 30%. Note that the take-off and landing distances will grow accordingly - the STOL in the aircraft's name will not apply any longer!

When putting a glove over the slot, the wing area will be maintained and the minimum speed will not go up quite so severely. Using the 34.6% from NACA TR 407, the minimum speed increase would only be 23.6%. The change in airfoil shape (with the glove the point of maximum thickness is further back and the relative thickness smaller than with the slat removed) will not alter the flight characteristics in a significant way - in both cases the airfoil is of the peaky upper surface type with a large nose radius, so separation around stall will start from the trailing edge and move forward with increasing angle of attack. Long laminar runs on the top surface cannot be expected in both cases.

Now I only wonder why you would add vortex generators. I thought the idea was to reduce drag and improve cruise performance?