Unlimited lives:
Enter I WANT TO CHEAT as a name at the high score screen. Then, begin game play and collect the small boat on the second screen. 

Cheat mode:
Load or reset the game, then execute the following BASIC commands before running or restarting the program. 

POKE 8026,173
SYS 12470

                          * MONTY ON THE RUN * - Solution

This is how to complete MONTY ON THE RUN. Select the following items first:

  No 2   - Jetpack
  No 4   - Rope
  No 12  - Passport
  No 13  - gas
  No 16  - bottle of rum.

Collect the coins you can but leave the ones which are dangerous. Always collect
the buns in The House. When you come round the first screen of the Sewerage
Works, walk through the teleport - this can be tricky - then collect the bun
that room. The wall will disappear as you can walk through. Collect everything
you see in this new area. When you come into the room with Help printed on it,
go into the teleport and you will find yourself in another room - collect the
switch and money. Make your way back to the Help screen and you'll see that the
floor has been filled in - but only if you've collected everything. Walk through
the teleporter and you'll find yourself in a room with a rocket pack. Move to
the right and you will eventually end up at the Tree Stump. Go to the top and
collect the object then descend using the jetpack and move left to "drive Sir
Clive's C5". You need the object from the Tree Stump to complete the game.
Move through the rooms until you reach the end and drop into Das Boat. Collect
all the objects, go to the bottom left-hand side of the boat and collect the
key. Now move to the top left-hand side of the boat and you will see a flashing
square. Jump onto this to finish the game.

Michiel, FidoNet 2:283/502

***
Check also out http://hem2.passagen.se/uffewald/monty for Monty on the Run hints.