1.) You do not compile ISConfig, you compile the apps shipepd with ispcnfig, like ispconfigs own apache 1.3 (inlcuidng mod_ssl and php5) as well as clamav, spamassassin etc.
1.a) PHP is an interpreted language (like perl) so there is no way / need to compile the php scripts.
2.) see 1.) or on what changes do you refer to any special "change"?
If you mean bugfixes etc. you can participaty and help buy checking ispconfig out via SVN and upload your updates via that. For more info about SVN ask till / falko