Whichever way you look at it, bolt on mods to get to you near 100bhp is going to be difficult ($$$) without forced induction or some other $$$$ bolt on mod.
A well ported head is worth 30bhp for sure (I'd argue more), but with conditions: Compression ratio (that means piston dish and type), cam profile, bore size, carb, exhaust and most importantly tuning. There are a lot of variables and depending on how your current motor has been built it, you may only realise some of the benefits before running into other problems like fuel starvation, oil starvation, diff issues, gearbox issues, etc etc. Brake issues, drive line issues, tyres, the list goes on.
It basically starts to bring out your car's weakest points. Example: If you have an unmodified crank/rods/flywheel setup and it has not been balanced/lightened then you will feel that the 100bhp that you have is not the same as an engine that has had this treatment, for example.
This is one reason why I decided to just buy another 1275 and rebuild that from the ground up. The cost will be similar either way, but at least you know what you put into it and how it was built. (HINT there is an Auto 1275 for sale right now in For Sale section for $250, from someone in QLD I think, bargain

)
Example: my engine now has a few problems, not major but they are there such as noisy lifter, gearbox problems etc. Suddenly a Quaife LSD doesn't sound too far off, and my Yoko A008's just don't cut it anymore. This is what happens when you are running 100+ bhp! it's not a simple matter of bolt on and go, it's a fine tune to get it to the edge and a fine tune to keep it there.
100bhp is not one silver bullet, it is multiple pieces as part of a whole package... if you get my drift...
Hope that is some food for thought...
