You give Far too little information for us to even guess at what the "problem" may be. I doubt that you have real problem, but read my comments below, then get back to us and we'll get you sorted out :-)!
In any case, what makes you think that a 3º list is a problem at all?
90 litres of water in a tank, weighs just about the same as a grown man. A man moving from one side of the
boat to the other makes no significant difference to the trim of a five ton
boat.
A Tartan36C — "C" for "centerboard" — must necessarily have the
bilge partitioned by the
centerboard trunk. You do NOT have a "ballast space", you have a "bilge". The
bilge is NOT meant to hold ballast, It is meant to collect water that accidentally gets into the boat, and the boat should have a
pump - a "bilge pump" — to
pump that water out so you don't get wet feet, or anything else!
What you term "backboard" is called "port" in
English. "Backbord" and "Steuerbord" are perfectly correct terminology in German. In
English, it's "port" and "starboard".
What you say you have, is not a "heel". It is a "list". A boat that "leans to one side" when lying still is said to "have a list" or "to be listing". A sailboat that "leans to one side" due to the pressure of
wind in the
sails is said to "heel". A boat like yours will sail best when it is heeling 10 or 12 degrees rather than zero degrees.
Replacing an
engine, even with a quite different engine, does not make a boat list.
Your
mast is not "keel footed". It is "stepped on the keel" or, in other words, "keel stepped". The mast should, in the thwartships plane, be absolutely perpendicular to the boat's design waterline plane. If it isn't, you adjust your "standing rigging" ("shouds" and "topmast stay") until it is. The simple way to verify that it is, is to measure CAREFULLY the distance from the "truck" (the top of the mast) to the "toe rail" that runs along the
outboard side of the
deck. The points on each toerail, port and starboard, to which you measure need to be PRECISELY the same distance aft of the stemhead.
Bonne chance — Alles Gute :-)
TrentePieds