You would probably want to separate your numbers with some kind of delimiter like a space or comma or something so you can tell where one value ends and the next one starts.
For example, if you had HP$="1337" and SP$="42" you would end up with GAMESAVE$="133742", and there is no way to say you didn't start with HP$="133" and SP$="742".
You can then use INSTR to search for the delimiter.
For example, if you were using a comma:
START=0
COMMA=INSTR(START,GAMESAVE$,",")
HP$=MID$(GAMESAVE$,START,COMMA-START)
START=COMMA+1
COMMA=INSTR(START,GAMESAVE$,",")
SP$=MID$(GAMESAVE$,START,COMMA-START)
To illustrate, I assumed you put a comma after the final value. Otherwise you would need to use COMMA=LEN(GAMESAVE$) for the last one.
To decrease the amount of repeated code, we can write a function to help out:
DEF NEXT_TOKEN S$,I OUT TOKEN$,J
J=INSTR(I,S$,",")
IF J<0 THEN J=LEN(S$)
TOKEN$=MID$(S$,I,J-I)
INC J
IF J>LEN(S$) THEN J=-1
END
I=0
NEXT_TOKEN GAMESAVE$,I OUT HP$,I
NEXT_TOKEN GAMESAVE$,I OUT SP$,I
