above specs are good, but I would add does it need to be 10.000x ?
ie ask what short term, and long term deviation are allowed.

The simplest 8051 method of doing this, will be to capture a full cycle of Fi, and then divide that time by 20, and load into a Toggle-Pin timer.
Choose an 8051 variant with prescaled timers, and you can cascade some timers
to capture to 32 bits,
That will be close to 10x, and it you want a precise average, you would add code that calculated the two closest toggle pin values, one above 10x and one below.
You then continually count the 10x edges, and swap between the reload values, to give a long term average of 10.00.
Run a paced update, so the output always generates 10 cycles per IP reading.
This is less computation than DDS, and more precise long term.