Unfortunately in my case I have no idea of the positions of the lost characters, but I keep trying, because it has been very useful for me to learn new skills and resources.
In which case, I think the search space is something like (58chars*47positions)*(58chars*47pos)*(58chars*47pos)*(58chars*47pos)... (58*47)4 = 5.5220891*1013 possible combinations 
It should be possible to modify the code (or write new code) to test all the possible combinations... but it is going to take a LONG time to search all of that. Testing 1,000,000 combinations/second will take nearly 2 years to search all of them. (for reference, my Core i5-3570K, running Ubuntu on the Windows 10 Linux Subsystem seems to average around 200,000 combinations/second.)
Not exactly "impossible" (and for 5 BTC, probably worth it
)... but technically it will be difficult to run a task non-stop for that period of time.