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
If you stay in Nairobu there are several used parts located in industrial area east of th city. A b14 engine will fit but make sure you use a b12 tranny and axles. you will also need the engines ecu(brain box) and wiring harness. also make sure to match up the displacement of the b14 engine to your current one i.e 1300 cc for 1300 cc. eg ga13de for e13s, ga15de for e15e, e15s ga15e ga15s. if not you will have to get the b14 tranny as well. that will also be the case if your b12 came with CA16/18de.