Here's a very basic spreadsheet to track your quantities and running totals of *stuff*. It's my lance, but this'll let you see all the various components and so forth and you can just relabel stuff for your lance if you prefer. Cells do have calculations in them, so be aware that it's possible to break equations.
Since it doesn't actually seem to be in TechManual, replacement locations (such as a 45 ton Right Arm) are a separate line item. The cost is (240*Mech tonnage) in C-bills. A replacement Head is a completely separate item, and I can't recall the equation (I'm pulling those prices out of AtB), so if you find it in TM, please cite the page. Everything else is straight out of the book. Remember, a shoulder or hip actuator hit requires you to replace the entire location, which is why the actuators don't appear in the listing, as they're included for free in the replacement limb.
And since it WILL come up, remember that using your Techs to pull parts and armor off of destroyed units is absolutely a thing. PCs are expected to make a reasonable attempt at an even split of salvage ("you take the Jenner since you have a 30-ton Mech in your lance, since I have two 50-tonners I'd like that dead Centurion"). If there's a dispute, or if/when people aren't reasonable, the PC commander for a particular mission will split up salvage as they see fit.