Tips on how to improve your Mehari:
Some of the things are only suited for drumbrake models!
Imrovements also suited for disc brake models are labeled with: (Disc).

For better cabin ventilation (sometimes this is a problem, 
say on hot rainy days when the upper half doors can't be removed):
Install the fan shroud from a disc brake model (2CV or Dyane) and 
connect the discbrake cooling outlets with your dash mounted vents.
This way you have plenty of fresh cold air, even at idle.

For better demist/heating: (Disc)
Connect the dash mounted vents with your heatexchangers.
There are 2 ways of doing this: 
a) Connect a y-piece at each heatexchanger.
b) Cut a hole on the outside of exchangerbox, weld a pipestud on, 
and connect to the vents.

Comments: 
a) is the easiest way, but will not allow you to have warm air
in the vents and no heat on your feet. 
b) Allows independent control of the dash vents, 
but the only way to turn the heat off the vents, will be to 
close the vents, and hoping that the vents are tight.

Easier heat control:
Fit another control lever under the dash, remove the "synchro cabel" 
between the 2 heater boxes, and route a new cable to your 3rd control 
lever. Now you can adjust your heat with much more accuracy, controlling
one box per cable.

Better heat distribution:
Block the driver's side hole in the heat distribution box, then drill holes
or cut out a section of the box. Looking at box, you will notice a shaft
from one side to the other, this is the flapshaft. You can do anything you want 
below this shaft, but not above, as demisting will suffer. So perforate the plate
facing the cabin, (only the bottom half, under the shaft), and heat will be 
distributed more evenly. You can also do as i did, and fit louvres facing upwards,
now the hot air will distributed up and under the dash, eliminating the cold draught.

Safer wiper operation (Disc, also for 2CV and HY): 
(Caution, only if your car is fitted with the old style "pepperbox" wipermotor).
Add another lightswitch behind your turn indicator switch, connect the lightswitch
to the wipermotor, and if you have electrical washer, connect this to the horn switch.
If you stumble upon an old fashioned switch for twotone horns, connect the first "horn"
to yor wipers, and the second "horn" to the washer. A light dab on the switch, and wipers
sweep once, a heavier dab and wipers and washers goes on. Very neat!

Better hillclimbing and acceleration, and for superb towing ability:
Fit an 2CV4 or AZU gearbox. Just fit your large drums and it fits, naturally providing, that
you have spring coil clutch and not diaphragm clutch. In the latter case, adaption is easy, just
another flywheel and clutch unit.
You will now have max torque at app.70 km/h in fourth, and max power at around 105km/h.
110 km/h will be reached at 6000 rpms. Neither topspeed nor consumption will suffer, as you won't
use 3rd as much as with the Mehari box, and since the topspeed of the Mehari box is very sensitive 
to the direction of the winds, your average speed will be the same, if not go up.
This mod can be done to discbrake Meharis as well, but is more tricky, because you will have to strip
the discbrake gearbox, and fit the guts of the AZU/2CV4 box..

Concerning engine tuning, take a look on Jeroen Cats' page.
I currently use a A06/664 engine, it is not the most powerfull of the 602cc range,
but it runs 95 octane unleaded, and that fact suits my wallet just fine.
And in combination with the AZU box, it performs really well.
(So well, that i can do very noticeable wheelspin in third gear under heavy acceleration,
and this seems to impress certain young people, that usually laughs when they see the car.)