You could install a ventilation fan with a built-in heater and light. I think Panasonic makes the best fans. The Panasonic FV-0511VHL1 is a good example. However the challenge will be pulling out the old fan and trying to fit the new one. Depending on the size it could be difficult. You will also need to run an additional wire to a switch if you want independent control for the heater, fan and light. It may be easier to install a wall mounted heater. You will get better heating, thermostat control, and many options to choose from.

yes, that is my question, how to avoid installing a ventilation fan since we don't need it. any suggestions for a wall mounted heater.

You want a heater that will provide enough heat on a cold night. How big is the bathroom? How cold does the bathroom currently get? Heater come with 120 V and 240 V wiring. If it is a small bathroom then the 120 V should be fine, but a big bathroom may need a bigger heater wired to 240 V.

bathroom is about 9 x 15, probably could get as low as 45 in the dead of winter but more likely around 50...

What is a Ventless Gas Heater?

Ventless gas heaters are room-heating furnaces that do not vent air to the outside of the house. Instead, the oxygen that is needed to fuel the combustion process for heat is taken from the air inside of the room in which the unit is placed, and the resulting heat is vented back out directly from the unit.

https://www.air-conditioning-heating.org/

Because there is no outside air exchange -- which is the reason most traditional heaters lose the majority of their heat -- ventless heaters are much more efficient than traditional heating systems, however, there are some drawbacks and limitations to be aware of.

FWIW, the OP has edited the size of the room.

The bathroom is 9 X 15 and currently has no heating? How is the rest of the house heated?

it is currently heated, marginally, with toekick heaters/forced hot water but we want a way to quickly heat it up when needed.

Get a 1500 watt portable heater - I like the oil-filled radiators for this purpose - and try it out. This will determine how much heat you need. Neither of my bathrooms have any heat and they stay warm enough if the door is left open when unoccupied. Long Island, NY

thank you for suggestions but I am can figure out btus' needed, wondering more about type of heater...i see envi mentioned, there is also steibel eltron and other brands....any suggestions?

If you have the space I suggest a wall heater. Cadet seems to have several models available at the big box stores. This Cadet 3000 BTU 240 V model should be adequate.