Hi all,
New user here in need of help.

I recently bought an Amstrad MEGA PC 386SX + Keyboard. I got it as part of a bundled deal I made at a retro game store. It was sold as not-working as the shop owner could not get it to turn on. I bought it partially out of curiosity (big SEGA fan) and as a restoration project. It probably worked out to costing about £160 which looking back on it may have been too much. My intention is to preserve this rare piece of history and find a good use for it in my home. I am an electrical engineer and have a general good understanding of computers but it has been a LONG time since I have worked with computers of this age so I would be grateful of help.

It came with:
-Keyboard
- Tower (spec 25Mhz, 4x1MB RAM, 40MB HDD)

I did an initial tear down, inspection and testing:

Condition:
- The transformer and fan inside the PSU have failed. (can not source spare transformer so ordered a small ITX PSU to convert to AT with a -5v regulator)
- Battery has leaked, causing some small damage to motherboard. Battery removed, board cleaned with IPA, Vinegar and Contact cleaner, damaged traces fixed.
- Very dirty. I fully dismantled computer and cleaned everything. I plan on retrobriting when I have the weather/time.
- Have not yet converted the new PSU so not sure if there are any other problems. I suspect the HDD will be dead.

I would like to upgrade this computer but not stray too far away from the original spec, but if I could get hold of a PC7486SLC motherboard for a reasonable price I would consider that.

Proposed Upgrades:
- Overclock to 30Mhz or PC7486SLC motherboard if I can get it for a reasonable price.
- CF 512MB HDD, SD card reader on same IDE but with card slot cut into back of case for easy transfer of files
- Or CD-ROM drive on slave IDE
- 4x4MB SIMM RAM
- 512KB video RAM (upgraded from 256KB)
- Maths Co-Processor (optional)
- Windows 3.1 and 95 on separate partitions
- ISA LAN card
- 60Hz mod on Mega Drive ISA card
- Soundcard mod/upgrade

Questions:
- Can an IDE->SD and IDE->CF work on the same IDE socket? The IDE->SD card adapters I have seen don't appear to have a master/slave selector. The CF ones do. I would also like the CF adapter to be master, that is where I will install the OS. I plan to have an external SD card socket for 'non-plug and play' file transfer between PCs. The other issue is: Does this BIOS support 2 HDD on 1 IDE socket? There is only 1 4-pin molex connector on the PSU. Though this is not accurate indicator.
- Can the BIOS handle a CD-ROM drive on IDE?
- Can the motherboard handle 25 to 30Mhz overclock? Should I add a heatsink?
- Is there issue with using cheaper OSCs for the CPU clock? There seems to be quite a range in price between 60Mhz 4 pin OSCs. (£8.50 MEC on eBay, £1.50 unbranded on AliExpress)

I am currently away working abroad for 10 weeks but would like to get all the parts sorted before my return.

Any help or advise on this computer would be greatly appreciated.