Whitespace, 333 bytes

Try it online!

This is the same code as I posted to "Anarchy golf" ( http://golf.shinh.org/p.rb?Quine )

To be a bit more readable, substitute space,tab,newline to "s","t","n" like below;

ssstssttttststtsstsstsstsssssttssttsssssssttsstsssttsststtsttstttttsttttstttststtssttsttssssstsssstttttttsstststtststssstttsssssttsssstssssststssttsssttstsststtstsstttsttssssttttttsststtstssssssstsststtttssnsssnsnsnstsnnstsnsnsssstsnnstnsssttnnssnsntstsstntstssnsntstnsnsstsstsnnstnnsstnsnttsttnsssnsstttsssntsttssstsssssntssstnssntn

or disassembled code with my original interpreter/disassembler;

push +7969228294440588444206358371410407764689382769860480534316220(203b)  # 0000: ssstssttttststtsstsstsstsssssttssttsssssssttsstsssttsststtsttstttttsttttstttststtssttsttssssstsssstttttttsstststtststssstttsssssttsssstssssststssttsssttstsststtstsstttsttssssttttttsststtstssssssstsststtttssn
push +0(0b)   # 0207: sssn
dup           # 0211: sns
call +0(0b)   # 0214: nstsn
call +0(0b)   # 0219: nstsn
dup           # 0224: sns
push +2(2b)   # 0227: ssstsn
call null     # 0233: nstn
push +3(2b)   # 0237: sssttn
mark null     # 0243: nssn
swap          # 0247: snt
copy +1(1b)   # 0250: stsstn
div           # 0256: tsts
dup           # 0260: sns
jzero -0(0b)  # 0263: ntstn
dup           # 0268: sns
copy +2(2b)   # 0271: stsstsn
call null     # 0278: nstn
mark -0(0b)   # 0282: nsstn
swap          # 0287: snt
mod           # 0290: tstt
mark +0(0b)   # 0294: nsssn
push -24(5b)  # 0299: sstttsssn
mod           # 0308: tstt
push +32(6b)  # 0312: ssstsssssn
add           # 0322: tsss
putc          # 0326: tnss
ret           # 0330: ntn

The key idea is to decode the same encoded number as binary digit for the 1st time, and as ternary digit for the next, with recursive operations like a Python code below;

def decode(x,b):               # b=2 or 3
  x //= b
  if x != 0:
    decode(x,b)
  print("stn"[x%b], end="")    # "s" for 0, "t" for 1, "n" for 2

This code decode a number as a binary or ternary digit, then add 0 at the top and remove the last digit, finally substitute 0,1,2 to "s","t","n" respectively.

For example, if you want to encode a Whitespace code "sstn", use an even number 502=200121(ternary)=111110110(binary). At the 1st decoding as binary, "stttttstt" is created, and the 2nd decoding as ternary, "snsstn" is created. Finally, concatenate "ss" and these, the decoded code will be "ssstttttsttsnsstn", which is "ssstttttsttsn" ( "push 502" in Whitespace ) + "sstn" and thus what you want.

In addition, the conversion from 0,1,2 to s,t,n ( ASCII 32,9,10 ) is implemented with a formula y=mod(x,-24)+32.