The current through the SCR is limited by the output of the 555, so it is <0.5 A (555's source or sink a max of 200 mA). Almost any small, general purpose SCR with a current rating of ≥0.5A should work. What do you have available? If you list them, we have help you select.
You need to:
1: Ctrl-x and Ctrl-v that transistor
2.: "Ungroup component" the new transistor
3: Edit each of the pins to add numbers
4: Pull a selection box around the transistor's parts
5 "Group component"
About 40 years ago I used many 2N5061 SCRs and today I remembered the part number. Digikey has almost 6000 in stock today and the minimum order is only one for $.62US.
Why do you want a real SCR when you are simply simulating the circuit, not building the circuit? The Sim program might not know anything about a real SCR.
ExpressPCB allows you to designate any pin with any number for any schematic symbol. Some symbols come with the pins numbered, and some do not. It allows you to match pin numbers in schematic symbols with the matching parts in the PCB development package. You can also add or change the pin designations on the PCB parts. The libraries of schematics symbols and PCB parts are not always specific. Examples would be TO-93 packages for NPN/PNP/FET/SCR/Regulators that do not have the same pin alignment. You can make one-for-one symbol/part custom libraries. Bug or feature?
I use Express PCB. I think KMoffat has the right idea. I have seen that sometimes the connections are not completed. Try highlighting the part and dragging it a bit. If all of the traces do not follow then the program did not connect them and thinks the part is just hanging there
I've seen that too. But even if the traces do stick to the pin, the pin can still lack a number. Then you will get a netlist error when you do a check netlist.
You need to:
1: Ctrl-x and Ctrl-v that transistor
2.: "Ungroup component" the new transistor
3: Edit each of the pins to add numbers
4: Pull a selection box around the transistor's parts
5 "Group component"