I don't know really about HPI engines, so I cannot say if these can be considered as hop-ups, but one good, quite powerful and realiable (=easy) engine is the O.S. 15 CV-Hyper http://www2.towerhobbies.com/cgi-bin...?&I=LXPV46&P=7
. For more power although not so easy one is the O.S. 15 CV-R http://www2.towerhobbies.com/cgi-bin...?&I=LXALL4&P=7
for around $150. There's many shaft options though, so be careful to choose the right one for your car. I've used the CV Hyper for my Kyosho SuperTen along with Kyosho special tuned pipe and it's fast enough for me. For the coming summer I've got the 15 RX http://www2.towerhobbies.com/cgi-bin...?&I=LXBZ31&P=7
$159 which could be an option to you as well, but it may require more other parts to be installed on HPI (it comes only with the SG shaft).