toilet vent size issue

I posted this in remodeling - but it may be a better topic for this forum:

I am remodeling a powder room on first floor of two story 1925 house. Used to have a pedestal and a toilet on one side and a shower opposite. shower sucked and was not needed, sink and toilet were crowded. Took out all three, and am putting in a large vanity/Lav where the sink and toilet used to be; and putting a new toilet where the shower was. The D and W part of the DWV are pretty easy. Venting, though . . . There is a 2" vent line (easy to use) for the new Lav. There is a nice, straight vertical 1.5" vent that goes up through the roof and down to the crawl space, not 2 horizontal feet from where the new closet flange/toilet will be. I REALLY don't want to change that to 2" -- Very easy to hook up as is, totally a bear, maybe not even possible to change it. So -- can i put in one single toilet with a 3 inch drain and a 1.5 inch vent ?! Is it code and, more importantly, will it work? Thanks in advance.

Also - what alternatives? AAV in crawl space??

An AAV needs to be above the floor plane, you can't put it under the toilet. You could put in in the bathroom behind a removeable panel, but it must be accessable for repair and if there is a clog, above the place where it could leak.

yeah i am not a fan of AAVs anyway.
What about running about 10 feet "horizontal" in the crawl space over to the wall with the 2 inch line, going up and y-ing in to that vent (how far above sink does that have to be?) is it ok to have a long horizontal (properly inclined) before it goes up to the roof? thanks again.

The toilet can be vented within six feet.
Most codes require at least a 2" vent for the toilet.
Will 1.5" work? Yes.