My mobile home furnace Miller M1MB070A is outputting lukewarm heat and I am
Wondering what would it take to modify or adjust it to increase the heat output?
Would slowing down the blower speed be one way?

You could try slowing down the blower. But it might start tripping the limit switch. Have you looked at the flames?

You will only get the rated BTU X the efficiency rating. Make sure filters are clean and ductwork open. Insulate the main duct. Might be possible to put in larger orifices but that is an HVAC tech job.

The temperature rise spec on your furnace is 45-75*. Measure the temperaure of the air coming in. Take this measurement at the furnace door where the filter is mounted.Then measure the temperature at the nearest register.

Measurements must be taken while the furnace is running & has been for 5-10 minutes.

I would expect about a 50-60* difference. If so, you are within spec.