Use Douglas Crockford's json2.js library:

json on github

It restores that missing functionality by just including it into your source.

Before I get berated on not reading the question, let me rephrase:

Using the json2.js library, in my opinion (not that it matters), would not lead to a significant performance loss it is also not a lot of code to include especially when minimized:

if(typeof JSON!=="object"){JSON={}}(function(){"use strict";function f(e){return e<10?"0"+e:e}function quote(e){escapable.lastIndex=0;return escapable.test(e)?'"'+e.replace(escapable,function(e){var t=meta[e];return typeof t==="string"?t:"\\u"+("0000"+e.charCodeAt(0).toString(16)).slice(-4)})+'"':'"'+e+'"'}function str(e,t){var n,r,i,s,o=gap,u,a=t[e];if(a&&typeof a==="object"&&typeof a.toJSON==="function"){a=a.toJSON(e)}if(typeof rep==="function"){a=rep.call(t,e,a)}switch(typeof a){case"string":return quote(a);case"number":return isFinite(a)?String(a):"null";case"boolean":case"null":return String(a);case"object":if(!a){return"null"}gap+=indent;u=[];if(Object.prototype.toString.apply(a)==="[object Array]"){s=a.length;for(n=0;n<s;n+=1){u[n]=str(n,a)||"null"}i=u.length===0?"[]":gap?"[\n"+gap+u.join(",\n"+gap)+"\n"+o+"]":"["+u.join(",")+"]";gap=o;return i}if(rep&&typeof rep==="object"){s=rep.length;for(n=0;n<s;n+=1){if(typeof rep[n]==="string"){r=rep[n];i=str(r,a);if(i){u.push(quote(r)+(gap?": ":":")+i)}}}}else{for(r in a){if(Object.prototype.hasOwnProperty.call(a,r)){i=str(r,a);if(i){u.push(quote(r)+(gap?": ":":")+i)}}}}i=u.length===0?"{}":gap?"{\n"+gap+u.join(",\n"+gap)+"\n"+o+"}":"{"+u.join(",")+"}";gap=o;return i}}if(typeof Date.prototype.toJSON!=="function"){Date.prototype.toJSON=function(e){return isFinite(this.valueOf())?this.getUTCFullYear()+"-"+f(this.getUTCMonth()+1)+"-"+f(this.getUTCDate())+"T"+f(this.getUTCHours())+":"+f(this.getUTCMinutes())+":"+f(this.getUTCSeconds())+"Z":null};String.prototype.toJSON=Number.prototype.toJSON=Boolean.prototype.toJSON=function(e){return this.valueOf()}}var cx=/[\u0000\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,escapable=/[\\\"\x00-\x1f\x7f-\x9f\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,gap,indent,meta={"\b":"\\b"," ":"\\t","\n":"\\n","\f":"\\f","\r":"\\r",'"':'\\"',"\\":"\\\\"},rep;if(typeof JSON.stringify!=="function"){JSON.stringify=function(e,t,n){var r;gap="";indent="";if(typeof n==="number"){for(r=0;r<n;r+=1){indent+=" "}}else if(typeof n==="string"){indent=n}rep=t;if(t&&typeof t!=="function"&&(typeof t!=="object"||typeof t.length!=="number")){throw new Error("JSON.stringify")}return str("",{"":e})}}if(typeof JSON.parse!=="function"){JSON.parse=function(text,reviver){function walk(e,t){var n,r,i=e[t];if(i&&typeof i==="object"){for(n in i){if(Object.prototype.hasOwnProperty.call(i,n)){r=walk(i,n);if(r!==undefined){i[n]=r}else{delete i[n]}}}}return reviver.call(e,t,i)}var j;text=String(text);cx.lastIndex=0;if(cx.test(text)){text=text.replace(cx,function(e){return"\\u"+("0000"+e.charCodeAt(0).toString(16)).slice(-4)})}if(/^[\],:{}\s]*$/.test(text.replace(/\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g,"@").replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g,"]").replace(/(?:^|:|,)(?:\s*\[)+/g,""))){j=eval("("+text+")");return typeof reviver==="function"?walk({"":j},""):j}throw new SyntaxError("JSON.parse")}}})() 

You can drop it in using only one line and be done with it. Writing your own syntax for something that's already been done is going to take a lot of time away from doing the other work to finish your project. Any code that you will write will have to go through the same rigorous testing as this json library which has been time tested and proven to work.

This one line of code is a 3KB file.

json on github