Explanation
The best course of action to preserve business continuity for a very old PC that is running a critical, proprietary application in MS-DOS is to purchase matching hardware and clone the disk. This way, the technician can create an exact copy of the PC's configuration and data on another PC that has the same specifications and compatibility. This will ensure that the application can run smoothly on the new PC without any installation or configuration issues. Performing scheduled chkdsk tests would not help, as chkdsk is a tool that checks and repairs disk errors, but does not prevent hardware failures or software compatibility issues.
Upgrading the hard disk to SSD would not help either, as SSDs may not be compatible with the old PC or the MS-DOS operating system. Performing quarterly backups would help with data protection, but not with hardware availability or software compatibility. References:
https://www.howtogeek.com/199068/how-to-upgrade-your-existing-hard-drive-in-under-an-hour/
https://www.howtogeek.com/66776/how-to-repair-disk-errors-in-windows-7/