I would be highly suspicious of any odometer on a school bus that old that says it only has 31K miles.
On average, over the life of a bus, most buses put on 10K per year. In the early years perhaps as many as 20K but in the later years, particularly if they spend time on the spare line, less than 5K per year. 15 years=150K miles.
As it has been noted above, increasing the HP will not increase the top speed. Increasing HP will allow you to keep going the top speed without slowing down on any hill.
In order to go faster you will need to change the final gearing. The easy and cheap way to get about 5 MPH more top speed is to replace the current tires with taller tires. Another way would be to change the rear end gearing to a lower numerical number in order to get the top speed up. But in order to go faster you will most probably need more HP to do it. More HP can cause overheating problems, particularly on hills in hot weather.
Which is why I advocate very strongly to find a bus with the power package and gearing you want at the start. It may cost more than the one that is close by but you will save many $$$$$ over for what it will cost to update and upgrade a bus that won't travel at highway speed.
