Often, an amateur will look at the cost of a product and say "I cannot afford that, I will make it myself". In some cases, e. g. a PROM/Flash programmer, this will hold if you have plenty of time and do'nt value it much - or - just look for a learning experience.
To make a 'simple' non-optimizing "C" compiler for the '51 I would guess would be a task of something like 10-25 man-years.
If you look at the cost of "C" compilers and think a bit you will realize that this is not a task you even want to think about. When competition exist the products are priced based on the cost of making and maintaining them.
If you want to 'play' with such, go for an assembler, somewhere on the philips website there is an assembler ('51 or XA, I do'nt recall) in "C" source.

Have fun,

Erik