Copied from the rules thread to make it easy to find:
1. Scrapping a damaged limb/component - Mod: N/A, Time: 0 (per previous question thread)
Modifier: n/a
Time: n/a*. Removing a destroyed component or location doesn't require time or a roll. For example, a medium laser or hand actuator which takes a critical hit can simply be outright replaced (at the normal modifiers/time) and don't have to be removed prior to installing a new one. I feel the need to point out that hips and shoulders are the exception here, requiring *removal* of all intact components in the limb as per the normal rules, and installing a totally new limb. Note that you CAN scrap a limb without removing components, with the caveat that it'll scrap all the components, too. Scrapping the intact left arm of a Wasp 1A? You'll also lose the armor, and the upper arm, lower arm, and hand actuators.
2. Attaching a new limb after carefully removing/scrapping an old one - Mod: ?, Time: ? (blown off limb would seem harder and it's a +2, 180 minutes)
As it's not listed elsewhere on the Master Repair Table, it would quality as replacing Miscellaneous Equipment.
Modifier: +1
Time: 120.
3. Removing a head or side torso - Mod: ?, Time: ? (breaking down a mech for parts may certainly have intact pieces that can be removed)
As it's not listed elsewhere on the Master Repair Table, it would quality as replacing Miscellaneous Equipment.
Modifier: +1
Time: 120. *However, you have to remove EVERYTHING removable on the old location before removing it. Taking off the left arm of a Wasp 1A? Remove the armor, and the upper arm, lower arm, and hand actuators.
4. Attaching a side torso or head after a previous one carefully removed - Mod: ?, Time: ? (not sure if this counts as "destroyed" location for torso, or "blown off" for head)
As it's not listed elsewhere on the Master Repair Table, it would quality as replacing Miscellaneous Equipment.
Modifier: +1
Time: 120. *Attaching a new location only gets you the location, not the components in that location (as a corollary to the requirement above that everything attached to the location has to be removed prior to removing the location itself). Attaching the left arm of a Wasp 1A? You've just installed the internal structure and shoulder actuator; you will subsequently have to install the armor, and the upper arm, lower arm, and hand actuators.