Replacement for 2N6027
They share the same datasheet, and I think the differences almost certainly won't affect the experiment (oscillator?)
The only significant difference I can see is the peak current on the 2N6028 is lower, which means it will turn on a bit sooner. If set up as an oscillator I think this will just produce a lower amplitude waveform.
Disclaimer - I have only used one of these once a long time ago (almost unheard of nowadays) so I wouldn't put this in writing (er, hang on...)