You should be able to just replace the tubes, meaning both EL84s. A quick search on the web indicates that the Blues Jr is a fixed bias amp. This will allow you to simply pull the EL84s and replace with new ones. You should change both tubes and should order a matched pair. Different manufacturer's tubes will have slightly different characteristics. Some break up sooner, others later, have subtle differences in sound, etc. A number of folks on the board go to some high end tube dealers. The high end dealers will speak with you about the qualities you are looking for in your amp behavior and sound and make a recommendation. I myself have only used online resources like thetubestore.com and tubedepot.com to read about the various tube manufacturers and opinions on the characteristics, etc.
In short, for best results you need to replace both EL84s with a matched pair. The info on the web says the Blue Jr has a hot bias which is contended to shorten tube life. If possible you may want to try to chose a tube make that handles the hot bias best. Hot bias meaning basically too much current is fed to the tubes for the nominal operating characteristics of the tube type. This can cause early breakup in the amps power section as well as lead to shortened tube life.
Be careful removing the cracked tube as you don't want to have the glass bottle shatter when pulling it. Other than that, it is an easy thing to do. Hopefully others on the board can point you to the high end dealers, if you want to take that direction.
The river keeps a talkin'