I would change the sequence of LPT setting and video memory deletion, LPT would contain 3F left margin, after deleting video memory left margin would set to 0B, by this 52 sec problem, and dirt on the screen can be avoided if default EXOS system segment is used as video memory. The easiest way (and not 100% sure) if you check if page0 contains FC, if it is, then the machine is EP64. You do not use the extra memory, and after 52 seconds the original version would run on EP64, but after my patch it does not, it even does not use extra memory, but by changing video memory from FF to FD the game will overwrite 2nd game page by video memory, because it is loaded to FC and FD.

if you upload the source, i can add the mentioned modifications

It is a cool game. I played until level 10 and it was too hard for me.
I miss the sound effects.

Woah, that was some serious rom hacking.

Thanks, that made it simple to fix, I would have spent days trying to figure out the LPT page issue.

I've fixed that problem as well the header size byte. I've added some sound effects and updated the colours (I had the rgb colour bits around the wrong way).

The problem was that FF segment was used for Graphics page, EXOS LPT is located on FF segment, and the program deleted Graphich page 1st and then changed the LPT from EXOS LPT to Game LPT, so LPT pointer remained on empty segment, and took 52 seconds until it reached a "reload" bit somewhere in the memory.
I changed Graphics segment from FF to FD, 2 byte should be patched 1 where FF segment was paged in, and other at Graphics address in LPT.
Now the game starts immediately.