I flew around Ukraine in 2005 in my Robin, accompanying a Tiger Moth (v slow and we had to stop EVERYWHERE). We had the help of a local pilot who flew with us to do the radio, satisfy the navigator requirement etc.
There was only small quantities of unofficial avgas near Kiev. Otherwise mogas is used (they know which type is best). This worked ok in my O-360 and the Tiger.
The paperwork is horrific. Without much patience and local help it is very difficult.
We had no problems with fuel thefts.
We were made very welcome and the girls are pretty.