Is it OK if I make modifications to a WC19 wheelchair?

No, not without consulting the wheelchair manufacturer. Because the structural design and strength of a WC19 wheelchair and its various load-carrying components are critical to its successful crashworthiness performance, modifying any of the primary load carrying components, such as drilling holes or replacing parts, could compromise the crashworthiness of the wheelchair. Modifying non-structural parts of a WC19 wheelchair is less dangerous, but should be done while keeping in mind basic occupant-protection principles. Thus, modifications that reduce the presence of sharp and rigid components or improve the retention of add-on components are encouraged.