In the old days we used Asbestos string!
You can get 1/8" PTFE Square packing

You are correct, The O ring would make it cleaner to replace and to disassemble the engine to get to the packing section looks like no bog deal as it seems to have a overhang crank.
parking mare island string on the piston rod of the engine and packing material for a ring in the piston cylinder. The confiscation issue may be a bit more than other engines but this could be handled during the steam up process/ bringing the engine up to temp.
I have a Anthony Bohaboy 5/8X5/8 oscillator that uses packing material for the piston gland and for a piston ring. This engine was made in the late 40’s. The boat also uses a kitchen rudder.

Hi RonH
Just had a look at it and the steam exits out of the crankcase, normally expelled steam is used to provide a draught through the boiler. However if you are going to put this in a model you would need to consider condensate.
As for the seal it's more than likely a silicone O Ring or rolled teflon ( which makes it easy to replace). Mechanical packing something that small with gland packing is a painful task.

From what i have seen its an O ring and i think the exhaust exits the left side of the base.

I do not have one but looked up the engine on the PM Research site. I do like the packing gland on the n
Bottom of the cylinder where the piston rod goes in. Do not known if it is a packing material or O ring. To repack this with packing material is no big deal but to replace a O ring could be a problem as the engine would need to be torn down. Not sure about where the exhaust is. Looks like a enclosed crank. Engine looks like a good deal. I like it, should be fun and it is about $100 USD cheaper than the Grahm engine.

Should be good as a twin oscillating engine it shouldn't get stuck unless dead centre as it has a steam inlet per cylinder that slides across the cylinder face same as the old Mamod Engines. What I would worry about is the engines size at only 1/2" x 3/4" stroke 2 cylinder
Regards
Ian
IGME

A member of my boat club has one in his launch.Having watched it run seams to be a good engine for the cost.
Rick.