32bit version of tortoise svn
I've now installed tortoise svn (64bit) and this one also does not appear in the context menu
If you install the 32bit version of tortoise svn you will probably find that the context menu appears for you (untested for WinSCP but it worked for me in another program). You can safely install the 32bit version alongside the 64bit version, just install it under a different name "TortoiseSVN 32bit" or similar.