Stretch space characters horizontally

I'm using XeTeX to mimic another document, but I only have a similar font which I stretch using the extend and letterspace options.

My main problem is though, that my font's spaces (U+0020) are only about half as wide as the original font's. I don't want to use U+2000 to U+200A, because I want to be able to change the exact width without re-editing everything. I defined

\newcommand{\K}{\hspace{1.2ex}}

But that's very verbose to use: Hello\K{}World

What I'd really like is an option that allows me to define a factor to stretch spaces by (it may disable TeX's automatic stretching, I don't use justified text or even automatic line breaks anyway), but I only found those for vertical spaces…

This works with pdflatex but not with my versions (3.1415926-2.2-0.9995.2 TeX Live 2009/Debian and 0.9997.4 TeX Live 2010) of xelatex. So a solution not using microtype would be nice…

\documentclass{article}
\usepackage{lipsum}
\usepackage[tracking=true]{microtype}

\DeclareMicrotypeSet*[tracking]{my}{font = */*/*/*/*}
\SetTracking[spacing={1000,,}]{font = */*/*/*/*}{1}

\begin{document}
\lipsum[1]
\end{document}
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I don't relly understand how the "texlive" tag is relevant here. The question would be the same with MikTeX, for example. – mpg Jan 1 '11 at 12:58
I thought it was a problem with my XeTeX version, since updating to TeX Live 2010 solved another fontspec-related problem for me, but I remove the tag now – pascal Jan 2 '11 at 14:57