if you dont have a computer and are using an old school mpc2000, you can still make it work... i did it for years
first take the drum loop you want and take it to the sample trim page (shift + 5)
loop it so it sounds good: no clicks, nothing like that. to do this you might want to hit the fit button on the loop menu (f5) and then adjust from there. you may want to go back and look at the trim screen too, to help so you have the loop right
got the loop right? from the loop screen hit f1 (trim) and then f5 (cut). now go back to the loop screen and turn loop to off.
hit any pad. it should play the loop, but just once and then stop.
now hit the params button to get to the beat loop function. set it for four (im assuming your loop has four beats like (1)boom (2)click (3)boom-boom (4)click). now itll automatically tell you what the bpm of your loop is.
hit main screen and set your sequence for this bpm. now go to program and set the sound you just cut for pad a1. go back to main screen. hit step (f1). hit pad a1 (just once). hit main screen. go over to bars and set it for one bar (it'll ask you if you want to do this so say okay). now hit play. it should play the loop exactly how it was before, but now the sequencer is playing it.
are we on the same page? i know this sounds real basic and you probably already know how to do this, im jusy trying to make sure

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after you do this, hit program (shift + 5), then params (f2), go over to tune and click on open window. now hit a1. it should say tune = 0, tempo = xx.x bpm. now when you adjust the tune, up or down, it'll adjust the tempo as well. once you do this, go back and change the tempo of the sequence (on the main page) to the new tempo you set in program...
thats a real simple way to (slightly) speed up or slow down beats
of course it isn't the best way, but you should understand how to do that before you start to worry about time stretch or anything like that...
the next step is cutting the beat into segments by hand, but this post is too long already... cheers