What's the worst that can happen? Your desire to get things running quickly and to bodgily cut corners on a 10 cent diode stuffs your brand new board!
(Some of the replacement boards being sold seem to use logic level FETs for solenoid drives - but I don't know explicitly about yours, and these are significantly more robust than the transistors they replace. One of the boards I've read the specs on uses zeners to effectively turn off the FET with the reverse voltage but again I don't know if yours does. So do not read this paragraph as encouragement - rather use caution as advised below)
If you are desperate to fiddle around and I can understand that when you get a new toy, then I'd suggest that you do not connect the solenoid you are concerned about until you have it all sorted.
Without going into all the details, what you can do when you do replace the diode is look at a solenoid that is functional and you know is installed correctly. What you'll see is that one side of the coil has two wires connected to it - this will be the positive. The other side will have a single wire which is the wire to the driver board. Look at how the diode is orientated on this good coil and install your new diode the same way on the coil you are replacing. Which is just what ddstoys said.
