The Solenoid engine is designed to use a small solenoid that is widely available, but you may have suitable solenoids around or have access to other sources of new or used solenoids.
For the solenoid to work, it must be capable of having a stroke length of 10mm. It doesn’t need to give any power over this range, but the plunger must not fall out of the solenoid when pulled out by 10mm.
The drive board will work with voltages from 6-18V (more if you remove the regulator and use a split supply) and will drive solenoids of up to 2A.
Physically the solenoid is screwed to the frame with two m3 screws at 12mm centres. Of course if you would like to modify or re-drill pretty much any solenoid can be accommodated.