There is a slight ratio difference in the upper arms giving a higher spring rate. The rears are the same and most of the time a
softer spring rate at rear would be better so no value there as an upgrade. There is also the axle pin dry vs wet which is a
large press job. Hi-Los with a longer drive tube will be requires at rear as opposed to dry arms there is also different
hand brake cables. As for drilling the upper arms 5/16 to 3/8 is not a big bite with a sharp bit and a drill press.   Steve (CTR)

https://hydragasandhydrolasticservice.com/?fbclid=IwAR0wPCL2WcgY-O7bsYDO4p0O6rwmS65xbaQAv9AEQb3hN8dWhAXPgurVvq0

Have a look at the pins. I don't know what MM supplied. If the pin is too small add a bush, if too big machine it

There is a guy in the UK that re conditions the bags and replaces the hoses he is also on FB but I don't remember the company name off hand.

For repair of a wet system. try looking at the Austin America web site for some answers.  Todd Miller knows what he's talking about.

Can these pins be had? Or can the ones that came in the mini mania kit be modified to fit without modifying the wet arm?

Because of where the knuckle joint sits in the arm. The ratio is better than the dry. Historic racers here use them.

Do tell me more. 

Hi-Sorry, I have dry parts that may help with the conversion if you go that way. 
Michael

Would you have a hydro bag with hose attaced? Where are you located? I’m in Sebastopol California

Hi-I am starting to clean out some parts. Try not to modify the hydro parts some are very hard to find. Post what you need.  I may be able to send you a few things.

Instead of drilling or replacing the from wet arms why not get the pins made to suit? That's what we do here. The biggest plus is better handling with wet arms on dry suspension.

I understand the concern with modifications to parts but this car is not a pristine version of a car that needs a nuts and bolts restoration back to original. It has a later 1275cc +040 and. The front valance has been hacked and patched up, same with the front wing. I’m just looking to have a reliable car to have fun with and fix up over time. 

I read that before I posted about this, and to answer your earlier question, it’s the hi-low, cone, shock and shock mount kit.

I replaced the lines front to back, and it blew a line a few days later. I the car sat in a field in California for a few years, looks like some of that time it was actually under water half ways up on the wheels. I’m going through all of the systems to clean and rebuild. I just don’t want this system to keep blowing lines. Seems more reliable to convert. 

The elephant in the room. Why not just fix the hydro?

So I’ve purchased the cones, hi-lows, shocks and mounts kit. I did read that the rear stub axles needed to be replaced for proper installation. I’m hearing all of you and frankly feeling a little annoyed that a conversion kit wouldn’t come complete. 

so if I sourced the front upper arms from a dry suspension instead of drilling, and purchased the rear stub axles, I’d still need to modify the front subframe. This is sounding more expensive than I was expecting. GAH!! I just want to drive my car. 

Turnip, did you read this? https://www.minimania.com/SUSPENSION___Converting_from_wet_to_dry_1228
Even MM recommends switching the subframes and suspension components to dry ones before using their kit. 

I only had to deal with the rear since my front suspension is all custom, but I'm using the original Elf hydro rear arms with coil springs, along with a special spacer/spring perch I got from Graham at Heritage Garage. 

But Alex is right on the money. If I hadn't done all of the other custom work on my car, I would have left the hydro alone. 

STOP !

Aside from the other excellent advice regarding the modification/ruination of the subframes to accomodate the rubber cones (if you've gone that way) there are other things to consider...

Before you lay a spanner on the car, I've had a look at the Mania advertised kits and they won't work without additional bits.
https://www.minimania.com/part/MMKT0909/Classic-Austin-Mini-Hydrolastic-Wet-To-Dry-Suspension-Conversion-Kit
https://www.minimania.com/part/MMKT0902/Classic-Austin-Mini-Hydrolastic-Wet-To-Dry-With-Hi-lo-Suspension

Neither of these kits contains either replacement dry rear arms or replacement dry rear stub axles.

The rear dampers therefore have nowhere to mount on hydro arms.

https://www.minimania.com/part/2A7364/Classic-Mini-Stub-Axle-Left-Rear-Swing-Arm-Dry-Suspension-With-Shock-Mount

https://www.minimania.com/part/2A7351/Classic-Mini-Stub-Axle-Right-Rear-Swing-Arm-Dry-Suspension-With-Shock-Mount

At the front, the top arms will need removing and have the bump-stop hole enlarging to allow the fitment of the front damper mounting pin, or again the damper has nowhere to mount.

If you haven't started the mods, consider Whee's comments and avoid altering the wet subframe.

The Mk1-Forum has several articles on fixes for hydro problems.  Though I could not find the article on fitting repair (yet) I suggest you Google using the string " site:mk1-forum.net hydro ".  You'll find lots of articles that might prove useful regarding hydro repair and replace.