Interesting project to start programming in 8051 assembly.

I think it should fit in a 89C2051, with few external components. 89C2051 is capable to drive LEDs directly. So You can use P3 for the segments and two lines to drive a 4017 johnson counter (count and reset) for muliplexing the digits. Use transistors as buffers for the current can be 7 x 20 = 140 mA per digit.
use two lines from P1 for keyboard rows and connect each keyboard colomn to a common of the segments. That way you can read 2 x 9 = 18 keys, which must be scanned, preferably by the same interrupt routine that drives the multiplexed digits.

Instead of using 7 segment LED displays, it is possible to use a HD44780 based one line x 16 characters LCDisplay. This needs a minimum of 6 or 7 I/O lines, so in case of 89C2051 there are only 8 or 9 I/O lines left with which you should do the keyboardscanning, with or without additional external chips.

Have a nice project :)

Henk