Encase you haven’t managed to get it apart I have a Lehmann “Rusty’ Porter I just picked up 2nd hand.
I had to pull it apart to replace the gears and motor, (the previous owner did not give it the love it required!) once she’s up and running again a visit to the paint shop will be required (for obvious reasons

).
She is still in pieces waiting on the parts, but I have put the major bits together so you can see what mine is for your reference.
It may not be exactly the same as yours but this is what I remember doing to get mine apart.
Unscrew the 6 screws into the cab from underneath and remove the cab
Unscrew the 4 screws holding the water tank on, remove the tank.
Unscrew the 2 screws from underneath next to the cylinders (not the two holding the front bar)
The boiler should now pull off with a bit of a wriggle.
To get to the motor unscrew the 2 screws on top of the chassis.
OR
If you only need to get into chassis to lube it you only need to undo the 4 screws on the bottom of the chassis to remove the bottom plate.
Hope this helps
Alan