I've recently learnt about this R7RS implementation:

    https://github.com/false-schemers/skint

The blurb:

    Cheap and fast R7RS Scheme Interpreter

    SKINT is a portable interpreter for the R7RS Scheme programming
    language. It can be built from five C source files (10K lines of code)
    with a single command. There is no distributives or packages; just
    compile the source files with your favorite C compiler, link them with
    the standard C runtime libraries, and you're done. For some platforms,
    precompiled binaries are available

It seems to work... (-:

     ~/dump/skint
    🚀 make
    clang is detected
    x86_64 architecture is detected
    clang -O3 -DNDEBUG -D NAN_BOXING -c -o s.o s.c
    clang -O3 -DNDEBUG -D NAN_BOXING -c -o k.o k.c
    clang -O3 -DNDEBUG -D NAN_BOXING -c -o i.o i.c
    clang -O3 -DNDEBUG -D NAN_BOXING -c -o n.o n.c
    clang -O3 -DNDEBUG -D NAN_BOXING -c -o t.o t.c
    clang  -O3 -DNDEBUG -D NAN_BOXING -o skint s.o k.o i.o n.o t.o -lm

     ~/dump/skint
    🚀 ./skint --help
    SKINT Scheme Interpreter v0.4.9
    usage: skint [OPTION]... [FILE] [ARG]...

    Options:
      -v, --verbose                Increase output verbosity
      -q, --quiet                  Suppress nonessential messages
      -A, --append-libdir=DIR      Append a library search directory
      -I, --prepend-libdir=DIR     Prepend a library search directory
      -D, --define-feature=NAME    Add name to the list of features
      -e, --eval=SEXP              Evaluate and print an expression
      -s, --script=FILE            Run file as a Scheme script
      -p, --program=FILE           Run file as a Scheme program
      -V, --version                Display version info
      -h, --help                   Display this help

    '--' ends options processing. Standalone FILE argument is treated as a script.
    If no FILE is given, skint enters Read-Eval-Print loop (stdin>eval-print>stdout)

     ~/dump/skint
    🚀 ./skint
    SKINT Scheme Interpreter v0.4.9
    Copyright (c) 2024 False Schemers

    skint] (define mod truncate-remainder)
    (define leap-year?
      (lambda (year)
        (and (zero? (mod year 4))
             (boolean=? (zero? (mod year 100))
                        (zero? (mod year 400))))))

    skint]
    skint] (leap-year? 2004)
    #t

Just wanted to share for your information.