Yes, darn near any engine can fit into any car with the right fabrications. Sadly, it would take a TON of modifications to have this done. The B13 SER engine code is a SR20DE, which was never offered in a B12. I believe it has been done, but again, lots of modification. Go to srforum.com for all the info on that SR20 engine, what it comes in, what it can be put in, and if it can be put in. Good luck.
SR20DET will drop nicely in it, its a 2.0L turbo lump and can be found on eBay for about £1000
Whether or not a march fuel gauge will fit a B13 tank depends on the size of the march gauge. You should be able to find a march gauge that will fit on a B13 tank.
Yes.
No
If it is the same engine (the I4 or V6)
Yes it will fit in but it will not bolt up to transmission
YES
Yes a rb20det will fit
no
yes it can.
It will fit if the 2000 engine is a 3.0 L VQ30DE-K. They are almost identical engines
yes