A resettable fuse or circuit breaker might work. They come with various trip delays, so you might find something that will open the circuit back up in the right amount of time. I believe larger gauss guns actually use sensors to turn off the circuit once the bullet passes a certain point. By doing that you can add multiple stages to further accelerate the bullet.

I don't think any operating characteristics are going to be critical to that circuit, so you just need to meet or exceed the voltage and current ratings. https://www.digikey.com/en/products/filter/transistors-bipolar-bjt-single/276?s=N4IgjCBcoGwJxVAYygMwIYBsDOBTANCAPZQDaIALAAxwDMdIh1cFtVIAuoQA4AuUIAMq8ATgEsAdgHMQAX0IB2CohApIGHAWJkQtABxUArPE49%2BkIaMkz5IAExgqEaCD4CAqhLG8A8qgCyuOjYAK4iuIwgIQIAtgCCkTGSAjBU7LYOVLQqbhae3n6BwWERhNEWAGqJyRbUcoSZCC65IPm%2BAUGh4ZHlIDEA6tUSKXaG9SAAtHYqaqIhWiSQ5GMcsmtAA

If you do replace with concrete, drill a few holes in the edge of the existing concrete and use a sticks of rebar to tie together the old and new sections