Potential Upgrades of Engine and/or Tranny for 1969 911S

I have a 1969 911S Targa in better than average
condition. It's got about 135,000 miles on it, and
gets healthy doses of love regularly.

I'm afraid my 1st to 2nd synchos are blown to the
point that I need to rebuild or replace the original
901 tranny -- which has got me thinking about a
project that has been in my mind for a while. I know
that the 1972 and 1973 911S has the highly sought
after 2.4 powerplant -- better horsepower and torque
range. Eventually I think I would like to switch over
now that it is old enough that I don't have to smog
the car any more.

So, I am faced with two issues:

(1) Being cost-sensitive, can I start this process now
by installing a 1972 or 1973 (Model 915/12) tranny
while keeping my original 2.0S motor? What, if any,
modifications are necessary?

(2) What would be the best way of going to the 2.4S
(or 2.7RS) setup? Should I bore and rebuild my 2.0
block? Should I start with a solid used 2.4? If so,
does it matter whether I use a 2.4T, E, or S as a
starting point? What, if any, modifications will I
need when I go to the 2.4 (2.7RS) motor?

I would really appreciate the insight of your
experience.

Many thanks.