Crutchfield is a good resource for figuring out what will fit in the stock dash location. You are correct in that you cannot use a speaker where the tweeter extends above the plane of the speaker very much - it will interfere with the OEM grill. Per Crutchfield, 4" speakers fit in the dash. You can filter the speaker selection based on what fits (by make, model, year). I'm not sure if its 100% accurate, though for something as old as the MK1.

 

I have used Infiniti Reference 4" in the front. You can teek the speaker tabs a bit to get them to sit a lil lower. I haven't owned a MK1 since 2015. I had to tweak a couple things. Post up a pic of the speaker opening

 

I used infinity Kappa 4 inch speakers in the front. paired with a memphis audio amp and a cerwin vega self powered 10" sub, they fill the cabin with sound. Something that surprised me; I just switched phones from a samsung fold to a Moto G play 2024. The atmos app on the Moto made a HUGE difference in the sound quality. I absolutely could NOT get my sound adjusted properly with the samsung as my audio source. If you are going with a double din stereo, dont bother with any of the install kits. i bought 3 of them and none of them had the parts necessary. A professional audio store suggested buying a stock stereo on ebay and HOPING it had the mounting tabs on it. I wound up using plumbers tape to fashion brackets. My only other suggestion would be to be very conservative on your amp power. The stock alternator puts out 70 amps and is on a 50 amp fuse in the main fuse box. 20 amps is NOT a lot to play with for audio but you should be able to get some decent volume and bass with the right parts.

good luck