I was inspired by the work of Tom Stanton (Thanks Tom!).
Unfortunately I did not see a working multy-cylinder engine and since it was time to improve my drawing skills I thought I'd design and print my own.
After opting for a flat twin there was little extra effort in making a radial and since it's effectively a 2 stroke it could be self starting, so that's what I did ;)

The engine uses a couple of bearings, o-rings, airsoft ammo, springs, some 2mm rods and several m2 bolts.

The crank bearing has the following demensions:
15mm outside
6mm inside
5mm thick.
The rod bearings:
5mm outside
2mm inside
2.5mm thick

All bolts are m2 which is actually too small to tap nicely in the plastic.
24X m2x10mm
1X m2x12 (cranks)
6X m2x15
pushrods are 2x32mm
springs: 17mm length, 5,4mm outer dia 0,5mm wire thickness.
o-rings: 4.5mm inner, 8.5mm outer

I tried to make it have a 3x 2.5cc displacement (11mm stroke, 17mm bore). I've not used gaskets yet between the cylinders and heads which could improve efficiency. The cam was my first attempt, it seems to work good enough.

It's purring nicely at 1 bar:
https://www.youtube.com/watch?v=g_-GaT5-Jkg

And sounds better at 4 bar:
https://www.youtube.com/watch?v=pnACFU9a8LI

Some performance data (not very accurate):
prop 9x4 apc c2
1 bar: 930 rpm
2 bar: 2250 rpm
3 bar: 3000 rpm
4 bar: 3960 rpm
5 bar 4500 rpm
6 bar 5250 rpm at which point the prop became a little loose which drasticly decreased performance.

I've added a new crank. It uses an m3 bolt in the center to keep the 2 halves together. it will allow for a 12mm thick propellor or so and held up the 8 bar i threw at it.
It will spin a 13x8 prop at 3000 rpm at 6 bar and will go up some at 8 bar. I've seen 3250. No issues yet accept frozen fingers. It does move air..
It sounded more alive with the smaller prop, but more stable at these speeds. It's still plastic..