Siemens will do this conversion for you (I seem to remember they also gave me the software to do it from a PLC5). However the end result was atrocious and I ended up regretting not starting it from scratch and copying over some of the rungs and conditions rather than a straight copy.
The reason is that although the system will work as it did on the SLC, troubleshooting is really hard. TIA Portal is made to display the variable name prominently and not so much the comment on the variable. As such your code then becomes a mix of N, B and F files which don't necessarily have much relevance to the person troubleshooting.
The SLCs also don't make use of re-usable code which is something you can leverage in a Siemens PLC to write and test less code.
I'd look at a S7-1500... depending on the processor and program size, it's possible the 1200 will run the program, but it does limit the options and upgrades possible in the system greatly and may not even be enough for all the IO you need.