Looking pretty good! I like the music.

NeoHomeBrew (./1) :
There are two important functions missing in the game because the Neobitz C devkit doesn't support them.
The first ist the "Insert Coin" routine - does anybody know how to check for a coin throw-in ?

For MVS, you'll need to check the hardware credit locations every so often. I do this as a part of my credits display routine.

In home mode, keep a pair of bytes for storing the software side credits (since there's no backup RAM on home/AES).

NeoHomeBrew (./1) :
Second, I would like to save the highscores to the MVS Backup-RAM or to the Memory Card. Is there somebody who please could help me with it?

For MVS Backup RAM, the data is stored based on some values in the program's header.
For the NeoBitz kit, this should be named common_crt0_cart.s. The location of data saved to MVS backup RAM is specified at $107 in the header ("DEBUG_DIPS" in the common_crt0_cart.s I have). However, the size right after it is set to 0. You should probably increase this.

From there, it's a matter of writing to your chunk of RAM that you've carved out for saving. Be careful of the first two bytes being the debug dipswitches. (I forget exactly when the data is backed up to RAM, however...)

Saving on the Memory Card takes a bit more work, and I haven't fully tried it. The CARD article on the wiki has a basic overview of the functions. You'll need to enable card writes before sending a command and disable them after you're done (see Memory mapped registers for the regs).