If you are talking about going to the jabiru 3300, you will need:
new engine mount
electric fuel pump
reworked control cables
new cowling with reworked cooling
starter solenoid
starter button added to panel
location for voltage regulator
new prop
custom made cooling plenum for dowdraft or baffling for updraft
You will potentially need:
a custom exhaust made (if you don't like the stock jabiru pusher exhaust)
relocation of the battery from the nose to the firewall (Oddesey 625 recommended by my dealer)
The engine weighs 175 lbs, dry, without the muffler, but including electric start and 20 amp alternator and mechanical fuel pump, hoses and attachment hardware.
Electric Fuel pump, 2.5 lbs., extra wiring, 2 lbs., addtional pelunum weight 2 lbs. solenoid, 1 lb.for a total of 182.5 lbs.
The smallest o-235 is listed by Lycoming at 243 lbs. Of course that is with a starter and a large alternator, but even if you figure you started without a starter and a with very small alternator then I am adding electric start, picking up 5 horsepower, and losing about 30-40 lbs in the rear.
The weight savings is less from an O-200, but the horsepower gain is larger.