It would be nice, if someone could help me with a nice source/hint...

var
  startline : word; { The starting line of text }
  c1, c2 : word;    { The chunk numbers to use }

begin
  retrace; {i.e. wait for it}
  startline := position div 16; { 16 pixels vertically per line... change
this for qhatever font size you're using }
  c1 := startline mod 25;
  c2 := c1 + 1;
  move (chunk[c1], vidmem[0], sizeof(chunk));
  move (chunk[c2], vidmem[25], sizeof(chunk));
  SetVerticalScroll (position);
end.

Now obviously you can optimize this a little bit and reduce any flicker
which may come up by doing two things:

1. Convert your move()s from byte-size to word-size
2. Only call the last three lines when c1 doesn't equal what c1 was last time

As a final note, this doesn't check for the end of the file or anything,
as I am assuming that you're not letting the position scroll that far. :)