Actually, ASCII is a separate issue from the radix conversion.

ASCII is a code for representing printable characters in computer systems
http://www.asciichart.com/ascii_hexadecimal.html

So converting from a number to displayable text really requires two steps:

1. Determine the value of each digit required in the chosen radix;

2. Determine the ASCII code to represent that digit in the displayable output.

Fortunately, for digits 0-9, the conversion from the numerical value to the ASCII character code is trivial:

Character '0' has ASCII Code 0x30;
Character '1' has ASCII Code 0x31;
Character '2' has ASCII Code 0x32;
Character '3' has ASCII Code 0x33;

You see the pattern - the ASCII code for the digit n can be found simply by adding n to the ASCII code for '0'

unsigned char digit_value; // Numerical value of the digit, 0-9
unsigned char ascii_code;  // ASCII code for the character. '0'-'9'

:

ascii_code = '0' + digit_value;

Note the use of the character constant '0' rather than the magic number 0x30 - this helps to make it clear that we're talking about characters.
It also makes the code portable to any other character-coding scheme (provided that scheme also has its codes for 0-9 in sequence)

It is left as an excercise to the student to consider how this may be extended to give ASCII text representation in Hexadecimal notation...