Usually the motor is okay but the grease in the gearbox is all hardened. Before trying to find a replacement try disassembling the gearbox, cleaning out all the old grease, and regressing with new. Makes a big difference.
The strongest motors I have seen for the GTV 6 are the big black sealed Ducellier motors. They are a real hoss compared to the cheap smaller motors used on the Spiders. Much better torque, and a heavier case. Once you lube the bearings they seem to run fine. I drilled a small hole in the back end of the casing for lube access to the rear bushing, then oil soaked the front bushing with the motor vertical in my vise for a couple of days.
As for the gear drive, I have disassembled several of these for both Spiders and GTV 6, and have not yet found one to be dried out, with stiff grease. Every one has had soft lithium appearing grease packed into the casing, lots of it. You can drill a small access hole next to the worm and worm gear, to be able to spray lithium grease into that space and onto the gear teeth. In fact, if you look carefully and study how they're built, you can drill that lube hole with the gear drive mounted in the door of the GTV 6. No need to remove it and fight the cables, etc.
I did a post on this recently, with photos as I recall. Besides the obvious electrical circuit, relay and switches, the other items that affect window speed are alignment of the guide tracks, condition of the felt channels in the guide tracks, the friction of the belt weatherstrip on the glass, lube of the cables and pulleys, and cleanliness of the glass pane itself. I'll look for that post link.
Just installed power window relays and the results are impressive. Here is a working diagram. I have a short video of the working Windows. Pm your email address and I will send video and how I did it .
I helped a friend install Ongaro marine wiper motors in an old Toyota FJ 55. I can say they work great. There is a small computer that syncs the two motors. I have one on order for the rear of my Alfetta as I can’t seem to find the OEM Bosch
Before you replace your wiper motor try putting 12V from the battery (or 14V or whatever the running voltage is) directly to the motor. If it's significantly faster you need to clean the connections along the way and add a relay.
I went through this recently with my car. I did source a new motor - it sped things up a bit but not great.
If your motor is old I'd suggest replacing it with a new one or rebuilding it. There are some services on the internet that rebuild wiper motors for Corvettes and similar that have no issue doing the Bosch motors.
The relay upgrade made no difference for my car to the wiper motor or windows
What did make a difference- higher rpms (or so I think) and some rain-X!
I suspect if you source a wiper motor from a different (modern) car and do some wire hackjob, it may speed things up considerably but that is above my pay grade.