I'm running this conversion, I installed it about two months ago.  It is
pretty straightforward, but there are some things to consider and watch
out for.

The 2.1 has a larger intake plenum and throttle body, so use those, not
the 1.9's.  That will require a slight modification to the vacuum hose
routing as well.  Use the 1.9 exhaust, it is cheaper to replace anyway.
All the EFI components from the 1.9 should also be used, and take this
opportunity to replace the fuel lines if you have not already done so.
Ken Wilford has a great kit, see http://vanagain.com.

You will also need to use the water pump and thermostat housing from the
1.9, and throw in a new t-stat just for the heck of it. Don't forget to
use new gaskets, and replace those two short lengths of hose on the
engine, they are easy to do now.

 The last decision is the oil cooler, which won't fit or work with the
1.9 coolant plumbing and exhaust.  I have been running without it, but
after a long run on the freeway the oil pressure drops to 8-10 psi at
speed and 3 psi at idle, causing the light to go on.  I am going to
replace the PO's cooler set up that came with the engine to see if that
helps, but it may just be a worn engine.  This pressure drop is due to
heat, the pressure cold is 60 at idle and 75 at speed.  The heat both
thins the oil and expands the bearing clearance.  The 2.1 does run
hotter (more power = more heat), and the main bearings are cooled only
by the oil, so I may see some improvement with the cooler.  This
requires the addition of an external filter set up and tapping the
t-stat housing and water pump for a barbed hose fitting to get coolant
to the cooler.

I can give you more details if you want to do this, but first try
running without it. Just use the short oil filter nipple from the 1.9 in
the 2.1.  I strongly recommend the addition of the VDO oil pressure
gauge with the dual sender so you can monitor the condition of the
engine.

Have fun, it was a noticeable increase in power, especially since my 1.9
was on it's last legs at 205,000 miles!
--
Stuart MacMillan

'84 Westy
'65 MGB
'73 MGB GT