From the current 2017 IPSC rulebook:
16. Original parts and components offered by the OFM as standard equipment, or as an option, for a specific model handgun on the IPSC approved handgun list are permitted, subject to the following:
16.1 Modifications to them, other than minor detailing (the removal of burrs and/or adjustments unavoidably required in order to fit replacement OFM parts or components), are prohibited. Other prohibited modifications include those which facilitate faster reloading (e.g. flared, enlarged and/or add-on magwells, etc.), changing the original color and/or finish of a handgun, and/or adding stripes, stippling or other embellishments.
16.2 Magazines accessible to a competitor during a COF must not contain more than 15 rounds at the Start Signal. Identifying marks or decals, internal capacity limiters, bumper pads and additional witness holes, which add or remove negligible weight to/from magazines, are permitted.
16.3 Sights may be trimmed, adjusted and/or have colors applied. Sights may also be fitted with fiber optic or similar inserts.
17. Aftermarket parts, components and accessories are prohibited, except as follows:
17.1 Aftermarket magazines are permitted, subject to 16.2 above.
17.2 Aftermarket open sights (see Rule 5.1.3.1) are permitted, provided their installation and/or adjustment requires no alteration to the handgun.
17.3 Aftermarket grip panels which match the profile and contours of the OFM standard or optional grip panels for the approved handgun and/or the application of tape on grips (see Appendix E3a) are permitted. However, rubber sleeves are prohibited.
So no aftermarket triggers. Parts from the original firearms manufacturer that are available on a model that is in the IPSC approved handgun list (h ttp://www.ipsc.org/rules/proddiv.php) are ok. Replacing the factory sights with aftermarket sights are ok, as long as no modifications to the slide are required.