One additional source for canvas would be Bear Creek Canvas
Bear Creek Canvas | Pop-Up Camper Recanvasing Specialists | Spencer WI
The two mentioned canvas replacement companies are likely to be cheaper than a custom job as they have patterns for a vast array of models. They will also have the sunbrella, window vinyl and screen material all on hand in multiple colors and ready to go.
For the furnace it may be just as easy to measure the space you have available and then peruse the catalogs of the current manufacturers for one that will fit. They are pretty simple devices, and fitment for the space is probably the biggest issue. If your pup has heat vents as well as the front output then you will need one with an appropriately placed knock-out for the venting.
IMO, the BTU output for a pup isn't overly important; in cold weather you aren't going to heat it the same as hardwall trailer. We always used ours to take the chill out of the air in the morning and evening, but we dressed for the weather too. It is not reasonable to expect to heat a tent the same as an indoor space - trying to maintain a steady 70 degrees.