How to update Fortran 77 programs to more modern Fortran to improve readability?

I am attempting to update the following Fortran 77 program using more modern Fortran languages to improve its readability. Can anyone provide some suggestions? I tried using find and replace to change "IS" to a more readable variables, but that caused type cast problems. This task proved to be very tedious for other Fortran 77 programs much larger than this one.

C         3D ISING MODEL
C         Critical temperature TC = 4.5116=1/.22165
          DIMENSION IS(10,10,10), EX(13)
          REAL *8 R(1)
          DATA IS/1000*1/
          ITMAX=5000
          ISTART=4000
          L=10
          NR=1
          ISEED=768521034
          M=L*L*L
          DO 1000 K=1,24
          TR=0.05+(K-1)*.05
          T=TR/.221655
          MR=0.
          DO 3 I=1, 13, 2
3         EX(I)=EXP(-2*(I-7.)/T)
          DO 2 ITIME=1,ITMAX
          DO 1 K1=1,L
          K1P1=K1+1
          K1M1=K1-1
          IF(K1.EQ.1) K1M1=L
          IF(K1.EQ.L) K1P1=1
          DO 1 K2=1,L
          K2P1=K2+1
          K2M1=K2-1
          IF(K2.EQ.1) K2M1=L
          IF(K2.EQ.L) K2P1=1
          DO 1 K3=1,L
          K3P1=K3+1
          K3M1=K3-1
          IF(K3.EQ.1) K3M1=L
          IF(K3.EQ.L) K3P1=1
        IEN=7+IS(K3,K2,K1)*(IS(K3M1,K2,K1)+IS(K3P1,K2,K1)+IS(K3,K2M1,K1)
     &  +IS(K3,K2P1,K1)+IS(K3,K2,K1M1)+IS(K3,K2,K1P1))
          CALL GGUBS(ISEED,NR,R)
          IF(EX(IEN).LT.R(1)) GOTO 1
          IS(K3,K2,K1)=-IS(K3,K2,K1)
          M=M+2*IS(K3,K2,K1)
C          WRITE(*,*) M,ITIME
1         CONTINUE
          IF (ITIME.GT.ISTART) MR=MR+M
2         CONTINUE
          WRITE(*,*) FLOAT(MR)/1000./FLOAT((ITMAX-ISTART)),TR
          WRITE(1,*) FLOAT(MR)/1000./FLOAT((ITMAX-ISTART)),TR
1000      CONTINUE
          STOP
          END

        SUBROUTINE GGUBS(ISEED,NR,R)
        IMPLICIT REAL *8(A-H,O-Z)
        DIMENSION R(NR)
        DATA D2P31M/2147483647.D0/
        DATA D2P31/2147483648.D0/
        DO 7 I=1,NR
        ISEED=MOD(16807.D0*ISEED, D2P31M)
        R(I)=ISEED/D2P31
    7   CONTINUE
        RETURN
        END

I am still learning Fortran. I have a very old text book on Fortran applied to Molecular Dynamics with the following Fortran example. I was faced with the difficulty of learning a new language and translating Fortran help from new to ancient Fortran. I have spent time researching the problem and discovered that forcing non-implicit helps me to understand the program better. I am struggling with with some of Fortran's machine-like features like limited line space, character column syntax, and indenting the code to look neater without breaking the logic. In general, making the code look more readable without breaking the logic has been the greatest learning curve for me.

Solution

There are tools to convert FORTRAN 77 to Fortran 90 (converter software for fortran 77 to fortran 90). You might start with one of those. If the code will be continued to be used unchanged, then regard it as legacy code and don't tinker with it. If its not broken, don't fix it. If the code is going to be improved or expanded, porting to Fortran 90 will help the programmer. Improving from FORTRAN 77 to Fortran >=90 is an investment; balance how much investment to make versus how much programming work you want to do on the code. Make changes in stages and regression test the program to make sure that you don't introduce bugs. The work can be non-trivial. To have fully modern code you should remove the implicit declaration and declare all of the variables. Then you can rename some variables to have more meaningful names without concern for implicit typing.