It depends if if you have a good one or not. Some work Ok others not so good and still some really run great. They used to make the engines at maybe a dozen or more different locations inside Russia at the time. So the engines vary from one locale to another and as to whom was making them at the time too. Adrian Duncan of Model engine news discussed them here too
http://modelenginenews.org/ad/marz.html
You will likely need to replace the main bearings with better ones, replace the screws with better ones. Since there are only three screws holding the cylinder head on, this is important as the screws could break. Some people stabilize the crankshaft by heating it to 500 degrees and letting it cool slowly. They have been known to break crankshafts before.
Here is one of mine being limbered up:
