The MP7 uses Japan Mabuchi 370 class motor. In fact the 280s and the 370s are almost identical in terms of casing and diameter. The 280s are widely used in the RC Mini-T trucks. The 370s are considered the higher performance implementation of the 280s. These motors are in general very cheap (we are talking about several dollars a piece when buying in a QTY of 1000 or so).
When looking for replacement motors you need to pay attention to the voltage specification. For example, some 280s can take max 6V while some can take up to 12V. The 370s, however, are designed primarily for running (nominally) at 7.2V.
So is the stock 370 motor that comes with the MP7 powerful enough to pull an upgraded spring? The answer is yes, but you may see tradeoff in ROF. For a mildly upgraded mechbox (talking about some 5~7% or so improvement in FPS due to the use of spacer), the stock motor as sustained by the stock battery can still run fine. However, anything more than that may require increase in battery power (both voltage and mah) in order to maintain a reasonable ROF.
