How to represent a decimal number into a binary expression of a different radix/base

Disclaimer: this answer has a very limited scope to begin with.
Emphasis is on the method, which is solving for a polynomial equation of the form $$ aB^N+bB^{N-1}+cB^{N-2}+ \cdots + xB^2+yB^1+zB^0=X $$ For binary forms of size $2$ we trivially have, with $a\in\{0,1\}$ , $b\in\{0,1\}$ : $$ aB^1+bB^0=X \quad \Longrightarrow \quad B = \frac{X-b}{a} \quad ; \quad a \ne 0 \\ \Longrightarrow \begin{cases} ab = 10 & ; & B = X & ; & X = B \\ ab = 11 & ; & B=X-1 & ; & X = B+1 \end{cases} $$ One step more complicated, for binary forms of size $3$ we have, with $a\in\{0,1\}$ , $b\in\{0,1\}$ , $c\in\{0,1\}$ : $$ aB^2+bB^1+cB^0-X=0 \quad ; \quad a \ne 0 \\ \Longrightarrow \quad B = \frac{-b \pm \sqrt{b^2-4ac}}{2a} \\ \Longrightarrow \begin{cases} abc = 100 & ; & B = \sqrt{X} & ; & X = B^2 \\ abc = 101 & ; & B = \sqrt{X-1} & ; & X = B^2+1 \\ abc = 110 & ; & B = \left[-1+\sqrt{1+4X}\right]/2 & ; & X = B^2 + B \\ abc = 111 & ; & B = \left[-1+\sqrt{1+4(X-1)}\right]/2 & ; & X = B^2 + B + 1 \end{cases} $$ From this and the requirement that $X$ and $B$ must be integer, a number of examples can be derived, which is left as an exercise for the reader.

One other step more complicated.
For binary forms of size $4$ we have, with $a\in\{0,1\}$ , $b\in\{0,1\}$ , $c\in\{0,1\}$ : $$ B^3+aB^2+bB+c=X $$ Trying to solve for $\,B\,$ results in (computer) algebra you don't want to think about. Therefore I've decided to make a graph of the inverse of the polynomial, which is a zillion times easier than to determine the inverse algebraically.

A few cases are noticed. $$ \begin{cases} B = 0 & \Longrightarrow & X = 0,1 \\ B = 1 & \Longrightarrow & X = 1,2,3,4 \\ B = 2 & \Longrightarrow & X = 8,9,10,11,12,13,14,15 \\ B = 3 & \Longrightarrow & X = 27,28,30,31,36,37,39,40 \end{cases} $$ Continuing from there, the pattern becomes pretty boring.
Extending the method to $\,aB^N+bB^{N-1}+cB^{N-2}+ \cdots + xB^2+yB^1+zB^0=X\,$ seems prohibitive when done algebraically, and not very promising either.

Brute force approach

Free accompanying (Delphi Pascal) source code is available for this problem.
program later;
function binary(number,base : integer; var S : string) : boolean; var m : integer; bit : byte; OK : boolean; begin m := number; OK := true; S := ''; while not (m = 0) do begin bit := (m mod base); if bit = 1 then S := S + '1'; if bit = 0 then S := S + '0'; m := m div base; if not ((bit=1) or (bit=0)) then OK := false; end; binary := OK; end;
procedure reverse(var S : string); var k,L : integer; c : char; begin L := Length(S); for k := 1 to (L div 2) do begin c := S[k]; S[k] := S[L-k+1]; S[L-k+1] := c; end; end;
procedure for_number(X : integer); var k : integer; S : string; OK : boolean; begin { for k := 2 to X do } for k := 3 to X-2 do begin OK := binary(X,k,S); if OK then begin reverse(S); Writeln(X:5,k:5,S:10); end; end; end;
procedure loop(N : integer); var K : integer; begin for k := 1 to N do begin for_number(k); { Writeln; } end; end;
begin loop(100); end.
In Pascal, comments are enclosed in curly brackets. The commented for-do loop gives many trivial outcomes. Because of the base $2$ in the first place. And, with a number $X$ every base $B$ with $B=X$ and $B=X-1$ will give rise to a binary, $10$ and $11$ respectively. If we skip these most obvious trivialities, then the output becomes something like this. The first column is the number $X$, the second column is the base $B$ and the last column is the $(0,1)$ representation with base $B$.
    9    3       100
   10    3       101
   12    3       110
   13    3       111
   16    4       100
   17    4       101
   20    4       110
   21    4       111
   25    5       100
   26    5       101
   27    3      1000
   28    3      1001
   30    3      1010
   30    5       110
   31    3      1011
   31    5       111
   36    3      1100
   36    6       100
   37    3      1101
   37    6       101
   39    3      1110
   40    3      1111
   42    6       110
   43    6       111
   49    7       100
   50    7       101
   56    7       110
   57    7       111
   64    4      1000
   64    8       100
   65    4      1001
   65    8       101
   68    4      1010
   69    4      1011
   72    8       110
   73    8       111
   80    4      1100
   81    3     10000
   81    4      1101
   81    9       100
   82    3     10001
   82    9       101
   84    3     10010
   84    4      1110
   85    3     10011
   85    4      1111
   90    3     10100
   90    9       110
   91    3     10101
   91    9       111
   93    3     10110
   94    3     10111
  100   10       100