These guys are all correct!
"Modular Technology" is at play here.
Same engine in all versions (except the Marauder), different PCMs, trannys', gear ratios, as well as add ons or omitions of certain parts like oil coolers or speed sensitive steering.
Go and find yourself a PCM for a P71 and go from there.
Remember, a P71 motor only puts out 20hp more than yours.
Faster take off can be attributed to the lower gear ratio.
If your looking for 300hp like what's in a Marauder, well, go and get yourself a Marauder. All the mods you'll need to get yourself there will add up to the cost of one in the end.
Good luck in your quest.